If you're a Vanguard customer (buying the flagship fund classes, I presume), you're expressing faith in CAPM[0] and EMH[1] on a significant level. Some active investors will probably in the long term scoop up some extra gains, but the question still remains: which of them, over what time horizon, and is that a risk you want to take? Vanguard gives you lots of no-commission funds with 1 day liquidity even in the mutual funds - plenty of room for you to try and time the market if you think you can have a go at active trading around some predictable flows if too many people are passively investing. Nothing stops you.
[0]http://www.investopedia.com/terms/c/capm.asp [1]http://www.investopedia.com/terms/e/efficientmarkethypothesi...