It helps Vanguard because they don't make money through active trading: they are literally the vanguard of the movement, now practically accepted as orthodoxy, that funds should be passive and diversified across the whole market.
If you buy and hold, HFT helps you, by reducing the cost of execution --- both by reducing the spread, which is a tax you pay any time you place a market order, and by literally reducing trading fees. In fact, even if you're an active trader, HFT usually helps you: the only people truly harmed by HFT are the ones who tried to make a living selling liquidity before, who are now being outcompeted.
