Order-handling companies pay for "dumb" flow. Vanguard can reduce their outright trading costs to negative by being as dumb about it as possible, and then use these negative costs to artificially lower their reported fees. Just because Vanguard claims to be smart about it, doesn't mean necessarily they actually are incentivized to be smart about it or actually are in practice. People can still judge them by how close…
Is there any evidence that Vanguard gets kick-backs in return for their dumb order flow? I would think that would be a HUGE scandal if it were true and ever came out.
http://vrs.vista-one-solutions.com/reports/1-6/vang/
Short answer is that Vanguard Brokerage Services did not receive compensation for equities order flow in 1QTR 2015, but did receive compensation for options order flow.
I have no idea what the laws are surrounding their index funds or how those index funds are allowed to/actually do interact with Vanguard Brokerage Services.