As I mentioned in a few child comments, I don't think Vanguard can implement tax loss harvesting in a way that's customer facing at all. They certainly can't implement it in the way that Betterment does - that would involve purchasing competing mutual funds' ETFs, which I'm sure the SEC would view as anticompetitive.

I suppose you could do something like that within a sector - trading an Amazon for an Apple in tech, or a Shell for an ExxonMobil in oil - but that's betting on a lot of independent factors besides the performance of the commodity or sector as a whole.