Asia - Angkor from SGN, how long?

Witht eh current cheap fares to SGN, I'm thinmking of using that as a base to get to Angkor and PHN. Assuming no more time in SGN than necessary- the inbound flight arrives around 11 at night, and the outbound leaves around midnight, would 7 nights "on the ground" be enough to travel to both places, and not feel TOO rushed?

Also, I know the Vietnam visa on arrival can only be gotten at an airport. Once recieved, does such a multi-entry visa allow re-entry through a land border?

If you apply for a multi-entry visa, then yes it will be good for land border re-entry (the correct term is a pre-arranged Visa. You can't just get a VOA without pre-arranging it with a travel agency).

I spent 4 days at Angkor and felt that was about right. So 7 days total for both places would work for me.