Vanguard Target Date funds rebalance automatically.
> including asset selection
Vanguard's asset selection is "literally buy everything on the market". Its a dumb strategy, but it seems to work. In particular, Vanguard's total market index will perform by definition the average (minus Vanguard's very low fees).
> how to manage taxes
Its no harder than Betterment. You get a 1099-DIV next year, and then fill out your taxes. Since Vanguard Target Date funds automatically rebalance and everything, its unlikely that you get any benefits from Betterment.