VMBIX is a mutual fund offered by Vanguard that focuses on investing in mortgage-backed securities. These securities are debt instruments that are backed by a pool of mortgages, providing investors with a steady stream of income. As an institutional fund, VMBIX is designed for larger investors and aims to track the performance of a specific mortgage-backed securities index. This fund offers a diversified portfolio of mortgage-backed securities, providing investors with a low-cost and efficient way to gain exposure to this asset class.