Material Event (Form 8-K)

As previously disclosed, on May 21, 2025, Vigil Neuroscience, Inc., a Delaware corporation (the "Company" or "Vigil"), entered into an Agreement and Plan of Merger (the "Merger Agreement") with Sanofi, a French société anonyme("Parent" or "Sanofi"), and Vesper Acquisition Sub Inc., a Delaware corporation and a wholly-owned subsidiary of Parent ("Merger Sub"), pursuant to which, Merger Sub will merge with and into the Company (the "Merger"), with the Company surviving the merger as a wholly-owned subsidiary of Parent. On June 30, 2025, the Company filed a definitive proxy statement (the "Definitive Proxy Statement"), as such may be supplemented from time to time, with the Securities and Exchange Commission (the "SEC") in connection with, among other things, the Merger.

Certain Litigation

In connection with the Merger, from June 18, 2025 to July 28, 2025, the Company has received 16 demands, including one demand made pursuant to Section 220 of the Delaware General Corporation Law seeking certain books and records of the Company, from purported stockholders of the Company (the "Demands"). The Demands generally allege that the Definitive Proxy Statement omits certain material information and seeks additional disclosures in the Definitive Proxy Statement.

The Company cannot predict the outcome of the Demands or any potential litigation. The Company intends to vigorously defend against the Demands and any subsequently filed complaints or similar actions that may be filed. It is possible that additional demand letters and/or complaints may be received arising out of the Merger between July 28, 2025 and consummation of the Merger. Absent new or significantly different allegations, the Company will not necessarily disclose such additional filings or demand letters.

The Company believes that the disclosures set forth in the Definitive Proxy Statement comply fully with all applicable laws and that no further disclosure is required or necessary under applicable laws and/or in response to the Demands. Nevertheless, in order to moot the disclosure-based claims asserted in the Demands and avoid nuisance and possible expense and business delays, the Company has determined voluntarily to supplement certain disclosures in the Definitive Proxy Statement with the supplemental disclosures set forth below (the "Supplemental Disclosures"). Nothing in the Supplemental Disclosures shall be deemed an admission of the legal merit of the Demands described above, or of the necessity or materiality under applicable laws of any of the disclosures set forth herein. To the contrary, the Company specifically denies all allegations in the Demands that any additional disclosure was or is required or is material.

SUPPLEMENTAL DISCLOSURES

The following supplemental disclosures should be read in conjunction with the Definitive Proxy Statement, which should be read in its entirety as supplemented. The inclusion in this supplement to the Definitive Proxy Statement of certain information should not be regarded as an indication that any of the Company or its directors, affiliates, officers, or other representatives, or any other recipient of this information, considered, or now considers, it to be material, and such information should not be relied upon as such. Defined terms used but not defined herein have the meanings set forth in the Definitive Proxy Statement. For clarity, new text within restated paragraphs from the Definitive Proxy Statement is highlighted with bold, underlined text, and deleted text within restated paragraphs from the Definitive Proxy Statement is highlighted with strikethrough text.

The disclosure under the heading "The Merger-Opinion of Centerview Partners LLC-Analysis of Merger Consideration" is hereby amended and supplemented by replacing the second paragraph of that subsection on page 60 of the Definitive Proxy Statement in its entirety with the following:

"For analytical purposes, assuming that for one CVR, the holder thereof receives a payment equal to $2.00 upon the achievement of the Milestone, based solely on the assessments of the Company's management as to the probability of realizing the Milestone Payment (as defined in the CVR Agreement) and the estimated timing of achievement of the Milestone, as reflected in the Projections (see the section of this Definitive Proxy Statement titled "The Merger-Certain Unaudited Prospective Financial Information"), and discounting the probability-adjusted Milestone Payment under the CVR Agreement back to the valuation date of June 30, 2025 using the midpoint of a range of discount rates from 15.0% to 16.5% based on Centerview's analysis of the Company's weighted average cost of capital, which was calculated using the Capital Asset Pricing Model and based on considerations that Centerview deemed relevant in its professional judgment and experience, taking into account certain metrics including target capital structure, levered and unlevered betas for comparable group companies, tax rates, market risk and size premia, Centerview calculated an illustrative risk-adjusted net present value for one CVR of $0.14."

The disclosure under the heading "The Merger-Opinion of Centerview Partners LLC-Discounted Cash Flow Analysis" is hereby amended and supplemented by replacing the second full paragraph of that subsection on page 61 of the Definitive Proxy Statement in its entirety with the following:

"In performing this analysis, Centerview calculated a range of equity values for the shares of Company common stock by (a) discounting to present value, as of June 30, 2025, using discount rates ranging from 15.0% to 16.5% (based on Centerview's analysis of the Company's weighted average cost of capital, which was calculated using the Capital Asset Pricing Model and based on considerations that Centerview deemed relevant in its professional judgment and experience, taking into account certain metrics including target capital structure, levered and unlevered betas for comparable group companies, tax rates, market risk and size premia) and using a mid-yearconvention: (i) the forecasted risk-adjusted, after-taxunlevered free cash flows of the Company over the period beginning on July 1, 2025 and ending on December 31, 2043, as set forth in the Projections, (ii) an implied terminal value of the Company, calculated by Centerview by assuming that (as directed by Company management) the Company's unlevered free cash flows would decline in perpetuity after December 31, 2043 at a rate of free cash flow decline of 50% year over year, (iii) tax savings from usage of the Company's estimated federal net operating losses, as set forth in the Projections, and (iv) the net impact of assumed equity raises in 2025 and the period from 2026 through 2030, as set forth in the Projections, and (b) adding to the foregoing results the Company's cash balance of $67 million (including investments) as of June 30, 2025, as set forth in the Internal Data."

The disclosure under the heading "The Merger-Opinion of Centerview Partners LLC-Other Factors" is hereby amended and supplemented by replacing the first and second sub-bulletsof that subsection on page 62 of the Definitive Proxy Statement in its entirety with the following:

The disclosure under the heading "The Merger-Certain Unaudited Prospective Financial Information" is hereby amended and supplemented by replacing the third full paragraph of that subsection on page 65 of the Definitive Proxy Statement in its entirety with the following:

"At the direction of the Board of Directors, the Company's management calculated unlevered free cash flow as set forth below, which calculation has been reviewed and approved by the Board of Directors for reliance upon and use by Centerview in connection with the rendering of its opinion to the Board of Directors and in performing the related financial analyses as described in the section of this proxy statement titled "The Merger-Opinion of Centerview Partners LLC." For purposes of calculating the discounted cash flow, the Projections assumed (i) estimated federal net operating losses of $319 million as of June 30, 2025 and estimated future losses and (ii) the net impact of assumed equity raises of $80 million in the third and fourth quarters of fiscal year 2025 and $1,300,000,000 $1.3 billion in fiscal years 2026 through 2030 estimated by the Company's management based on the Company's projected capital requirement of approximately $1.2 billion on a non-riskadjusted basis and excluding proceeds from potential milestone payments andincluding a 10% discount anda 5% spread and a 10% discount (with such discount calculated on a grossed up basis to result in proceeds of $1.3 billion in the aggregate, prior to deduction of the 5% spread, over such period).

The disclosure under the heading "The Merger-Interests of Company Executive Officers and Directors in the Transactions-Potential Future Parent Arrangements" is hereby amended and supplemented by replacing the paragraph of that subsection on page 70 of the Definitive Proxy Statement in its entirety with the following:

"Certain of the Company's executive officers may continue to provide employment or other services to Parent after the Effective Time and may enter into new agreements, arrangements or understandings with Parent to set forth the terms and compensation of such post-Effective Time service. As of the date of this proxy statement July 28, 2025, no such agreements, arrangements or understandings with Parent exist."