VIMSX tracks the MSCI US Mid Cap 450 Index, which has a larger median market cap than the S&P 400, meaning that it is slightly skewed towards the large end of the mid cap spectrum. The larger market cap stocks in the MSCI Mid Cap 450 Index overlap slightly with the large cap S&P 500. Therefore, when large cap stocks outperform, VIMSX will outperform the S&P 400. When small cap stocks outperform, VIMSX will underperform the S&P 400.

Vanguard's Large Cap Index fund tracks the MSCI Large Cap Index, the Mid Cap Index (VIMSX) tracks the MSCI Mid Cap Index, and the Small Cap Index tracks the MSCI Small Cap Index.

Vanguard's 500 Index (VFINX) tracks the S&P 500, and Vanguard's Tax Managed Small Cap Fund (VTMSX) tracks the S&P 600 Small Cap Index. Vanguard does not have a fund that tracks the S&P 400 Mid Cap Index, but this index is tracked by the iShares Mid Cap ETF (IJH).

Vanguard's 500 Index fund (VFINX) tracks the S&P 500