Vanguard also provides equivalent ETFs - VOO has expense ratio .04%, vs VFINX .14% - basically the ETF is like the admiral class shares ($10k+ investment). https://advisors.vanguard.com/VGApp/iip/site/advisor/investm...

Mutual funds: recurring investment of any dollar amount (fractional shares). Minimum initial investment required. Trade once per day, beware loads, fees, and expense ratios

ETFs: buy or sell one or more whole shares, trade throughout the day, beware trading commissions and expense ratios.