Vanguard Sector Bond Index Funds Prospectus

Prospectus Supplement Dated June 20, 2023

Important Changes to Vanguard's Frequent Trading Policy

The Board of Trustees of the Vanguard Funds have approved certain changes to the Vanguard Funds Frequent Trading Policy, applicable to trades having a trade date of June 21, 2023, or later, as follows:

Prospectus Text Changes In the Investing with Vanguard section, under the heading "Frequent-Trading Limitations," the following bullet points are added under "These frequent-trading limitations do not apply to the following": ? Certain transactions below dollar value or other thresholds specified by Vanguard. ? In-kind transactions to a shareholder's donor advised fund managed by Vanguard Charitable.

In the same section, the following text replaces corresponding similar text in its entirety under "For participants in employer-sponsored defined contribution plans,* the frequent-trading limitations do not apply to":

*The following Vanguard fund accounts are also subject to the frequent-trading limitations: SEP-IRAs, SIMPLE IRAs, certain individual 403(b)(7) Custodial Accounts, and Vanguard Individual 401(k) Plans.

? 2023 The Vanguard Group, Inc. All rights reserved. Vanguard Marketing Corporation, Distributor.

PS FTPA 062023

Vanguard Sector Bond Index Funds Prospectus

December 21, 2022

Institutional Shares Vanguard Short-Term Treasury Index Fund Institutional Shares (VSBIX) Vanguard Intermediate-Term Treasury Index Fund Institutional Shares (VIIGX) Vanguard Long-Term Treasury Index Fund Institutional Shares (VLGIX) Vanguard Short-Term Corporate Bond Index Fund Institutional Shares (VSTBX) Vanguard Intermediate-Term Corporate Bond Index Fund Institutional Shares (VICBX) Vanguard Long-Term Corporate Bond Index Fund Institutional Shares (VLCIX) Vanguard Mortgage-Backed Securities Index Fund Institutional Shares (VMBIX)

This prospectus contains financial data for the Funds through the fiscal year ended August 31, 2022. 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.

Contents

Fund Summaries

Financial Highlights

58

Vanguard Short-Term Treasury Index Fund

1 Investing With Vanguard

65

Vanguard Intermediate-Term Treasury

Index Fund

6 Purchasing Shares

65

Vanguard Long-Term Treasury Index Fund

11 Converting Shares

69

Vanguard Short-Term Corporate Bond

Index Fund

16 Redeeming Shares

71

Vanguard Intermediate-Term Corporate Bond

Index Fund

21 Exchanging Shares

75

Vanguard Long-Term Corporate Bond

Index Fund

26 Frequent-Trading Limitations

75

Vanguard Mortgage-Backed Securities

Index Fund

31 Other Rules You Should Know

78

Investing in Index Funds

36 Fund and Account Updates

82

More on the Funds

37 Employer-Sponsored Plans

83

The Funds and Vanguard

52 Contacting Vanguard

84

Investment Advisor

52 Additional Information

85

Dividends, Capital Gains, and Taxes

53 Glossary of Investment Terms

87

Share Price

56

Vanguard Short-Term Treasury Index Fund

Investment Objective The Fund seeks to track the performance of a market-weighted Treasury index with a short-term dollar-weighted average maturity.

Fees and Expenses The following table describes the fees and expenses you may pay if you buy, hold, and sell Institutional 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 table and example below.

Shareholder Fees (Fees paid directly from your investment)

Sales Charge (Load) Imposed on Purchases Purchase Fee Sales Charge (Load) Imposed on Reinvested Dividends Redemption Fee

None None None None

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

Management Fees 12b-1 Distribution Fee Other Expenses Total Annual Fund Operating Expenses

0.05% None 0.00% 0.05%

Example

The following example is intended to help you compare the cost of investing in the Fund's Institutional Shares with the cost of investing in other mutual funds. It illustrates the hypothetical expenses that you would incur over various periods if you were to invest $10,000 in the Fund's shares. This example assumes that the shares provide a return of 5% each year and that total annual fund operating expenses remain as stated in the preceding table. You would incur these hypothetical expenses whether or not you were to redeem your investment at the end of the given period. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

1 Year $5

3 Years $16

5 Years $28

10 Years $64

1

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 more taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the previous expense example, reduce the Fund's performance. During the most recent fiscal year, the Fund's portfolio turnover rate was 59% 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. Treasury 1-3 Year Index. This Index includes fixed income securities issued by the U.S. Treasury (not including inflation-protected securities, floating rate securities and certain other security types), all with maturities between 1 and 3 years.

The Fund invests by sampling the Index, meaning that it holds a range of securities that, in the aggregate, approximates the full Index in terms of key risk factors and other characteristics. All of the Fund's investments will be selected through the sampling process, and under normal circumstances, at least 80% of the Fund's assets will be invested in bonds included in the Index. The Fund maintains a dollar-weighted average maturity consistent with that of the Index. As of August 31, 2022, the dollar-weighted average maturity of the Index was 2 years.

Principal Risks The Fund is designed for investors with a low tolerance for risk, but you could still lose money by investing in it. The Fund is subject to the following risks, which could affect the Fund's performance, and the level of risk may vary based on market conditions: ? Income risk, which is the chance that the Fund's income will decline because of falling interest rates. Income risk is generally high for short-term bond funds, so investors should expect the Fund's monthly income to fluctuate accordingly. ? Interest rate risk, which is the chance that bond prices overall will decline because of rising interest rates. Interest rate risk should be low for the Fund because it invests primarily in short-term bonds, whose prices are less sensitive to interest rate changes than are the prices of longer-term bonds.

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