Summary Prospectus by Investment Company (Form 497K)

December 19, 2025

Summary Prospectus

Vanguard Mortgage-Backed Securities ETF

Exchange-traded fund shares that are not individually redeemable and are listed on Nasdaq

Vanguard Mortgage-Backed Securities Index Fund ETF Shares (VMBS)

The Fund's statutory Prospectus and Statement of Additional Information dated December 19, 2025, as may be amended or supplemented, are incorporated into and made part of this Summary Prospectus by reference.

Before you invest, you may want to review the Fund's Prospectus, which contains more information about the Fund and its risks. You can find the Fund's Prospectus and other information about the Fund online at www.vanguard.com/prospectus and http://personal.vanguard.com/us/literature/reports/ETFs. You can also obtain this information at no cost by calling 866-499-8473 or by sending an email request to [email protected].

 

The Securities and Exchange Commission (SEC) has not approved or disapproved these securities or passed upon the adequacy of this prospectus. Any representation to the contrary is a criminal offense.

Investment Objective

Vanguard Mortgage-Backed Securities ETF, an exchange-traded share class of Vanguard Mortgage-Backed Securities Index Fund (the "Fund"), seeks to track the performance of a market-weighted mortgage-backed securities index.

Fees and Expenses

The following tables describe the fees and expenses you may pay if you buy, hold, and sell ETF Shares of the Fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and example below.

Shareholder Fees
(Fees paid directly from your investment) 

Transaction Fee on Purchases and Sales	None*
Transaction Fee on Reinvested Dividends	None*
Transaction Fee on Conversion to ETF Shares	None*

Annual Fund Operating Expenses
(Expenses that you pay each year as a percentage of the value of your investment) 

Management Fees	0.01 %
12b-1 Distribution Fee	None
Other Expenses	0.02 %
Total Annual Fund Operating Expenses	0.03 %

Example

This example is intended to help you compare the cost of investing in the Fund with the cost of investing in other funds. The example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year and that the Fund's operating expenses remain the same. 

1 Year	3 Years	5 Years	10 Years
$3	$10	$17	$39

This example does not include the brokerage commissions that you may pay to buy and sell ETF Shares of the Fund.

Portfolio Turnover

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund's performance. During the most recent fiscal year, the Fund's portfolio turnover rate was 91% of the average value of its portfolio.

Principal Investment Strategies

The Fund employs an indexing investment approach designed to track the performance of the Bloomberg U.S. MBS Float Adjusted Index (the "Target Index"), which includes fixed-rate U.S. agency mortgage-backed pass-through securities issued by the Government National Mortgage Association ("Ginnie Mae" or "GNMA"), the Federal National Mortgage Association ("Fannie Mae" or "FNMA"), and the Federal Home Loan Mortgage Corporation ("Freddie Mac" or "FHLMC"). A mortgage-backed pass-through security is a fixed income structure that pools mortgage loans with similar characteristics into a mortgage-backed security. For purposes of constructing the Target Index, the Index Provider groups these pools into "pool aggregates" based on a variety of factors. To be included in the Target Index, pool aggregates must have at least $1 billion currently outstanding and a weighted average maturity of at least 1 year. Under normal circumstances, the Fund invests at least 80% of its net assets, plus the amount of any borrowings for investment purposes, in the bonds that make up the Target Index.

The Fund invests by sampling the Target Index, meaning that it holds a range of securities that, in the aggregate, approximates the full Target Index in terms of key risk factors and other characteristics. The Fund maintains a dollar-weighted average maturity consistent with that of the Target Index. As of August 31, 2025, the dollar-weighted average maturity of the Target Index was 7.4 years.

Principal Risks

As with any investment, an investment in the Fund could lose money over any time period. The Fund's share price and total return may fluctuate, potentially within a wide range. The principal risks of investing in the Fund are summarized below. Each of the following risks could affect the Fund's performance:

• General Market Risk. The markets in which the Fund invests can be affected by a variety of factors. These factors, which can be real or perceived, may include economic, market, political, and regulatory conditions and developments as well as local, regional, or global events such as wars, military conflicts, natural disasters, and public health issues. In addition, investor sentiment and expectations regarding these factors can also impact the markets. Different parts of the market, including different industries and sectors as well as different types of securities, may react differently to factors

that affect the market. These factors can contribute to market uncertainty, market volatility, and fluctuations in the value of the Fund's investments, thereby resulting in potential losses to the Fund over short or long periods.

• Investing in Bond Markets. The Fund may be impacted by the general condition of the bond markets and by factors that affect bonds and bond issuers. For example, as a general rule, bond prices and interest rates move in opposite directions. When interest rates rise, bond prices tend to fall, and when interest rates fall, bond prices tend to go up. Bond income also is affected by changes in interest rates. Interest rates can rise or fall for a number of reasons, including, but not limited to, central bank monetary policy, inflationary or deflationary pressures, and changes in general market and economic conditions. Changing interest rates, including, but not limited to, rates that fall below zero, could have unpredictable effects on the overall market and may expose the bond markets in particular to heightened volatility and potential illiquidity. The degree to which the Fund is impacted by certain bond market risks may vary based on factors disclosed in its principal investment strategies, such as the types of bonds in which it invests and the overall credit quality, average maturity, and/or average duration of its bond holdings.

• Interest Rate Risk. During periods of rising interest rates, bond prices overall may decline, which could result in a decline in the Fund's value. The prices of longer-term bonds are more sensitive to changes in interest rates than the prices of shorter-term bonds.

• Income Risk. During periods of falling interest rates, the Fund's income may decline. The income paid by shorter-term bonds is subject to a higher degree of fluctuation than the income paid by longer-term bonds.

• Credit Risk. Credit risk refers to the chance that an issuer will default (fail to meet its credit obligations) or fail to make payments in a timely manner, which could result in a loss to the Fund. In addition, negative perceptions of an issuer's ability to make payments can cause the price of a security to decline. While all debt securities are subject to credit risk to some extent, those with higher credit quality ratings generally pose less credit risk than those with lower credit quality ratings.

• Bond Liquidity Risk. If the Fund is unable to sell a security at an advantageous time or price, its returns may be reduced. There may be limited trading in the secondary market for certain debt securities, which could make them more difficult to value or sell.

• Prepayment Risk. Certain bonds are subject to risks associated with prepayment. With respect to mortgage-backed, asset-backed, and similar debt securities, prepayment typically refers to borrowers repaying their debt early (e.g., before the maturity date). Prepayment of bonds held by the Fund would result in the Fund losing any price appreciation above the amount repaid. In addition, because prepayments occur more frequently in low

interest rate environments, the Fund likely would be forced to reinvest the proceeds from any prepayments at a lower interest rate than when the prepaid bonds were purchased, resulting in a decline in the Fund's income and a potential loss in the value of the Fund's investments. Frequent prepayments and subsequent reinvestment of the proceeds also would increase the Fund's turnover rate.

• Extension Risk. During periods of rising interest rates, certain bonds held by the Fund may be paid off substantially more slowly than originally anticipated. As a result, the value of the bonds may fall, resulting in a decline in the Fund's income and a potential loss in the value of the Fund's investments.

• Index Investing. The Fund is subject to risks associated with index investing. Because the Fund generally seeks to track the performance of the Target Index regardless of how the Target Index is performing, the Fund's performance may be lower than it would be if it were actively managed. Additionally, because the Fund does not hold all of the securities included in the Target Index, it is subject to the risk that the representative sample of securities selected by the advisor will, in the aggregate, vary from the investment profile of the full Target Index. The performance of the Fund's investments, in the aggregate, may not match the investment performance of the Target Index. This risk, known as tracking error risk, may be heightened during times of increased market volatility or under other unusual market conditions. The Fund also could be negatively impacted by changes to the Target Index made by the Index Provider or by errors made by the Index Provider. Any gains, losses, or costs associated with or resulting from an error made by the Index Provider will generally be borne by the Fund and, as a result, the Fund's shareholders.

• ETF Share Trading. Vanguard ETF Shares ("ETF Shares") are an exchanged-traded class of shares issued by certain Vanguard funds that represent an interest in the portfolio of securities held by the issuing fund. The Fund's ETF Shares are listed for trading on Nasdaq and individual investors may only buy and sell them on the secondary market at market prices. Although it is expected that the market price of an ETF Share typically will approximate its net asset value (NAV), there may be times when the market price of an ETF Share and its NAV differ significantly. Disruptions to creation and redemption transactions, the existence of significant market volatility, or potential lack of an active trading market for ETF Shares (including through a trading halt), as well as other factors, may result in ETF Shares trading significantly above (at a premium) or below (at a discount) the Fund's NAV or the intraday value of the Fund's holdings. Thus, you may pay more or less than NAV when you buy ETF Shares on the secondary market, and you may receive more or less than NAV when you sell those shares.

• Authorized Participants. Only Authorized Participants may engage in creation or redemption transactions directly with the Fund. The Fund has a limited number of financial institutions that may act as Authorized Participants.

The Fund's Authorized Participants are not obligated to engage in creation or redemption transactions. To the extent that the Fund's Authorized Participants are unable to or choose not to proceed with creation and/or redemption transactions with respect to the Fund and no other Authorized Participants step forward to engage in creation or redemption transactions with the Fund, the Fund's ETF Shares may trade at a discount to NAV and possibly face trading halts and/or delisting.

An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.

Annual Total Returns

The following bar chart and table show the Fund's historical performance and are intended to help you understand the risks of investing in the Fund. The bar chart shows how the performance of the Fund's ETF Shares (based on NAV) has varied from one calendar year to another over the periods shown. The table shows how the average annual total returns of the ETF Shares compare with those of a broad-based securities market index and one or more additional indexes with similar investment characteristics as the Fund. Keep in mind that the Fund's past performance (before and after taxes) does not indicate how the Fund will perform in the future. Updated performance information is available on our website at vanguard.com/performance.

Annual Total Returns - Vanguard Mortgage-Backed Securities Index Fund ETF Shares1   

1 The year-to-date return as of the most recent calendar quarter, which ended on September 30, 2025, was 6.76%.

During the periods shown in the bar chart, the highest and lowest returns for a calendar quarter were: 

	Total Return	Quarter
Highest	7.34 %	December 31, 2023
Lowest	-5.17 %	September 30, 2022

Average Annual Total Returns for Periods Ended December 31, 2024 

	1 Year	5 Years	10 Years
Vanguard Mortgage-Backed Securities Index Fund ETF Shares
Based on NAV
Return Before Taxes	1.43 %	-0.71 %	0.85 %
Return After Taxes on Distributions	-0.17	-1.72	-0.16
Return After Taxes on Distributions and Sale of Fund Shares	0.84	-0.95	0.22
Based on Market Price
Return Before Taxes	1.71	-0.67	0.87
Bloomberg U.S. MBS Float Adjusted Index (reflects no deduction for fees, expenses, or taxes)	1.54 %	-0.57 %	0.96 %
Bloomberg U.S. Aggregate Float Adjusted Index (reflects no deduction for fees, expenses, or taxes)	1.33	-0.27	1.39

Actual after-tax returns depend on your tax situation and may differ from those shown in the preceding table. When after-tax returns are calculated, it is assumed that the shareholder was in the highest individual federal marginal income tax bracket at the time of each distribution of income or capital gains or upon redemption. State and local income taxes are not reflected in the calculations. Please note that after-tax returns are not relevant for a shareholder who holds fund shares in a tax-deferred account, such as an individual retirement account or a 401(k) plan. Also, figures captioned Return After Taxes on Distributions and Sale of Fund Shares may be higher than other figures for the same period if a capital loss occurs upon redemption and results in an assumed tax deduction for the shareholder.

Investment Advisor

The Vanguard Group, Inc. (Vanguard)

Portfolio Manager

Joshua C. Barrickman, CFA, Principal of Vanguard and co-head of Vanguard's Fixed Income Indexing Americas. He has managed the Fund since 2013.

Purchase and Sale of Fund Shares

ETF Shares may only be bought and sold in the secondary market through a brokerage firm. The price you pay or receive for ETF Shares will be the prevailing market price, which may be more (premium) or less (discount) than the NAV of the shares. The brokerage firm may charge you a commission to execute the transaction. Unless imposed by your brokerage firm, there is no minimum dollar amount you must invest and no minimum number of shares you must buy. ETF Shares of the Fund cannot be directly purchased from or

redeemed with the Fund, except by certain authorized broker-dealers. These broker-dealers may purchase and redeem ETF Shares only in large blocks known as creation units, typically in exchange for cash.

An investor in ETF Shares may incur costs attributable to the difference between the highest price a buyer is willing to pay to purchase ETF Shares (bid) and the lowest price a seller is willing to accept for ETF Shares (ask) when buying or selling shares in the secondary market (bid-ask spread). Recent information about the Fund and its ETF Shares, including information on the Fund's NAV, market price, premiums and discounts, and bid-ask spreads, is available online at vanguard.com.

Tax Information

The Fund's distributions may be taxable as ordinary income or capital gains. If you are investing through a tax-advantaged account, such as an IRA or an employer-sponsored retirement or savings plan, special tax rules apply. You should consult your own tax advisor with respect to any particular U.S. or non-U.S. tax consequences of your investment in the Fund.

Payments to Financial Intermediaries

The Fund and its advisor do not pay financial intermediaries for sales of Fund shares.