The fund employs an indexing investment approach designed to track the performance of the CRSP US Mid Cap Index, a broadly diversified index of stocks of mid-size U.S. companies. Vanguard Mid-Cap Index's well-diversified, market-cap-weighted portfolio is a terrific way to invest in mid-cap stocks. Vanguard Mid-Cap Index Fund Admiral Shares (VIMAX) Also available as Investor Shares mutual fund and an ETF. Overview. The advisor attempts to replicate the target index by investing all, or substantially all, of its assets in the stocks that make up the index, holding each stock in approximately the same proportion as its weighting in the index.