Vanguard U.S. Stock Index Funds Mid …

1 Vanguard Stock Index Funds Mid- capitalization PortfoliosVanguard Extended Market Index FundVanguard Mid-Cap Index FundVanguard Mid-Cap Growth Index FundVanguard Mid-Cap Value Index FundAnnual Report | December 31, 2017 ContentsPlease note: The opinions expressed in this report are just that informed opinions. They should not be considered promisesor advice. Also, please keep in mind that the information and opinions cover the period through the date on the front of thisreport. Of course, the risks of investing in your fund are spelled out in the the Glossary for definitions of investment terms used in this the cover: Nautical images have been part of Vanguard s rich heritage since its start in 1975.

2 For anincoming ship, a lighthouse offers a beacon and safe path to shore. You can similarly depend on Vanguard to putyou first and light the way as you strive to meet your financial goals. Our client focus and low costs,stemming from our unique ownership structure, assure that your interests are paramount. Vanguard s Principles for Investing SuccessWe want to give you the best chance of investment success. Theseprinciples, grounded in Vanguard s research and experience, can put you onthe right Create clear, appropriate investment Develop a suitable asset allocation using broadly diversified Minimize Maintain perspective and long-term single theme unites these principles: Focus on the things you can believe there is no wiser course for any fund s Performance at a Glance.

3 1 Chairman s Perspective.. 5 Results of Proxy Voting.. 9 Extended Market Index fund .. 11 Mid-Cap Index fund .. 36 Mid-Cap Growth Index fund .. 60 Mid-Cap Value Index fund .. 78 Your fund s After-Tax Returns.. 99 About Your fund s Expenses.. 101 Glossary.. 104 Your fund s Performance at a Glance1 Total Returns: Fiscal Year Ended December 31, 2017 Total Returns Vanguard Extended Market Index fund Investor Shares Shares Market Price Asset Value Shares Plus Shares Select Shares Completion Index Core Funds Average Core Funds Average: Derived from data provided by Lipper, a Thomson Reuters Company.

4 Returns for the 12 months ended December 31, 2017, ranged from about 17% for Vanguard Mid-Cap Value Index fund to close to 22% for Vanguard Mid-Cap Growth Index fund . Vanguard Mid-Cap Index fund , which contains both growth and value stocks , returned about 19%. Vanguard Extended Market Index fund , which includes both small- and mid- capitalization holdings, finished with a return of about 18%. Each fund closely tracked its target Index . The Mid-Cap Growth Index fund was the only one that trailed the average return of its peers. Large- capitalization stocks outperformed their mid- and small-cap counterparts, and growth stocks outpaced value.

5 Health care, financials, and information technology were among the top-performing sectors on a relative basis for all four Funds . Other standouts included consumer goods and industrials. Energy detracted from each fund s Returns Vanguard Mid-Cap Index fund Investor Shares Shares Market Price Asset Value Shares Plus Shares US Mid Cap Index Core Funds Average Core Funds Average: Derived from data provided by Lipper, a Thomson Reuters Company. Vanguard Mid-Cap Growth Index fund Investor Shares Shares Market Price Asset Value Shares US Mid Cap Growth Index Growth Funds Average Growth Funds Average: Derived from data provided by Lipper, a Thomson Reuters Company.

6 Vanguard Mid-Cap Value Index fund Investor Shares Shares Market Price Asset Value Shares US Mid Cap Value Index Value Funds Average Value Funds Average: Derived from data provided by Lipper, a Thomson Reuters Company. Admiral Shares carry lower expenses and are available to investors who meet certain account-balance requirements. Institutional Shares,Institutional Plus Shares, and Institutional Select Shares are available to certain institutional investors who meet specific administrative,service, and account-size criteria. The Vanguard ETF Shares shown are traded on the NYSE Arca exchange and are available onlythrough brokers.

7 The table provides ETF returns based on both the NYSE Arca market price and the net asset value for a share. 6,879,964; 7,337,138; 7,720,749; 7,925,573; 8,090,646; and 8,417, the ETF Shares, the market price is determined by the midpoint of the bid-offer spread as of the closing time of the New York Stock Exchange (generally 4 , Eastern time). The net asset value is also determined as of the NYSE closing time. For more information about how the ETF Shares' market prices have compared with their net asset value, visit , select your ETF, and then select the Price and Performance tab. The ETF premium/discount analysis there shows the percentages of days on which the ETF Shares' market price was above or below the Returns: Ten Years Ended December 31, 2017 AverageAnnual ReturnExtended Market Index fund Investor Shares Completion Index Core Funds Core Funds Average: Derived from data provided by Lipper, a Thomson Reuters Index fund Investor Shares Mid-Cap Index Core Funds a benchmark description, see the Core Funds Average: Derived from data provided by Lipper, a Thomson Reuters Company.

8 Mid-Cap Growth Index fund Investor Shares Mid-Cap Growth Index Growth Funds a benchmark description, see the Growth Funds Average: Derived from data provided by Lipper, a Thomson Reuters Company. Mid-Cap Value Index fund Investor Shares Mid-Cap Value Index Value Funds a benchmark description, see the Value Funds Average: Derived from data provided by Lipper, a Thomson Reuters Company. The figures shown represent past performance, which is not a guarantee of future results. (Current performance may be lower or higher than the performance data cited. For performance data current to the most recent month-end, visit our website at )

9 Note, too, that both investment returns and principal value can fluctuate widely, so an investor s shares, when sold, could be worth more or less than their original RatiosYour fund Compared With Its Peer GroupInvestorSharesETFS haresAdmiralSharesInstitutionalSharesIns titutional Plus SharesInstitutional Select Shares Peer GroupAverage Extended Market Index Growth Value Index fund expense ratios shown are from the prospectus dated April 27, 2017, and represent estimated costs for the current fiscal year. Forthe fiscal year ended December 31, 2017, the Funds expense ratios were: for the Extended Market Index fund , for Investor Shares, for ETF Shares, for Admiral Shares, for Institutional Shares, for Institutional Plus Shares, and forInstitutional Select Shares; for the Mid-Cap Index fund , for Investor Shares, for ETF Shares, for Admiral Shares, for Institutional Shares, and for Institutional Plus Shares; for the Mid-Cap Growth Index fund , for Investor Shares, for ETF Shares, and for Admiral Shares.

10 And for the Mid-Cap Value Index fund , for Investor Shares, for ETFS hares, and for Admiral Shares. The peer-group expense ratios are derived from data provided by Lipper, a Thomson ReutersCompany, and capture information through year-end groups: For the Extended Market and Mid-Cap Index Funds , Mid-Cap Core Funds ; for the Mid-Cap Growth Index fund , Mid-CapGrowth Funds ; for the Mid-Cap Value Index fund , Mid-Cap Value Shareholder,Lately I ve been asked a lot about my legacy. And candidly, that phrase bugs me it isn t my legacy. Vanguard s success is a result of thousands of my colleagues committing to a clear mission: helping investors achieve their financial reason the topic has come up is that on December 31, 2017, I officially stepped down as Vanguard s CEO after almost a decade in that role.