Sure, plenty of businesspeople and such get new visas for every trip. In many countries (although not necessarily Vietnam), a record of several used single entry visas is actually required before they'll give you a multiple entry.

And you can probably get that Vietnamese visa much faster in SEA than in the US. I'm not sure about Phnom Penh, but both Singapore and Bangkok can do it in a few days at normal rates, and on the same day if you pay express surcharges.