I haven't offered any, and it would most likely be illegal if it was explicitly going on. And there is likely a wall between the different trading desks (though often the physical embodiment portion of this is literally a cubicle wall the employees can hear each other over). But the orderflow compensation doesn't need to cross over into the retail desk if the ETF itself has enough trading volume to mask some kickback without appearing too egregious.
But price fixing is also illegal--nevertheless, two gas stations across the street at a profitable intersection can engage in it solely through price signal tit-for-tat[1]. This effectively masks intentionality.
Much more fantastical and speculative: machine learning algorithms at both firms could now, or in the future, arrive at this cooperative strategy, even with the only communication being through price signals. Without any human ever even knowingly giving the explicit go-ahead.
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.
I would think that would be a HUGE scandal if it were true and ever came out.