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Because our company's 401K plan has limited choices. I'd go for index funds and/or ETF's if offered. BTW, my S&P 500 is an index (SWPPX).

mutual funds can be either index or actively-managed. Vanguard offers many mutual funds, many of which are index funds.

Vanguard funds have ridiculously low costs

Yes. I surveyed blogs written about index fund. Most suggested Vanguard (inventor of Index Fund). Like you probably noticed above, my ideal 3-fund portfolio consists Index Funds/ETF's from Vanguard.