> There are no commissions when you buy and sell low-cost Vanguard mutual funds and ETFs.
> If you buy and sell the same Vanguard ETF® in a Vanguard Brokerage Account more than 25 times in a 12-month period, you may be restricted from purchasing that Vanguard ETF through your Vanguard Brokerage Account for 60 days.
That said, mimicking could entail more than 25 transactions per ETF per year. I don't know that we really have the ability to predict the number of trades that the robo-advisor will do from the outside though.
You're definitely correct, but also Wealthfront recommends Vanguard ETFs disproportionately for their value [1]. A glance at my personal portfolio tuned to the higher end on their risk scale shows ~30% Vanguard ETFs.