Traveling to Vietnam with Dual Citizenship

I'm sure this has been asked before but I can't find a straight answer. I have a close friend who recently became a US Citizen and wants to visit her parents next year. But she isn't sure what passport to use. She became a citizen through marriage so both pasaports have different last names. If she travels with the US passport, is she supposed to get a Visa to show on her way out of the US that she can enter Vietnam legally? Can she use both passports, one to exit/enter the US, and one to enter/exit Vietnam? Would having exit/entry stamps on two passports for the same trip bring any issues? Thanks in advance for your help.