Vietnamese are friendly to Americans and see America as an ally in their struggle against Chinese imperialism. The hatred of China in Vietnam is near-universal and explicit.
I've lived there but if you don't believe me, ask a few Vietnamese yourself, they're not shy about it.
I actually met couple of Vietnamese (not in Vietnam though). They were not very fond of Americans to speak softly. No real hatred either (well they're very polite and I do not really now what their actual feeling were). I do believe that modern Vietmamese people are tolerant and welcomed to a tourists including the Americans. Tourists are income and people that actually did suffer war effect and immediate aftermath are mostly dead now.
Also the US did not bring "the piece and prosperity" to Vietnam unless you mean withdrawing from that murderous war that they started in a first place. What brought a relative prosperity was that their communist government did soften their stance towards market based economy. And feeling some guilt US probably contributed few pesos.
I've lived there but if you don't believe me, ask a few Vietnamese yourself, they're not shy about it.