Vietnamese never really hated the US, in fact Ho Chi Mihn the leader of the vietcong drafted a constitution that mirrored our own and was open to the western powers, they wanted independence but the French.
When the US soldiers were in the south they were generally friendly with the locals its not till later when the vietcong would hide among the populace did the US soldiers become more wary of the vietnamese but in the mind of the US soldier they were their to save the people, the people were not their enemies.
Many vietnamese live here in the US and have family back in Vietnam exposing them to American culture
Vietnam is currently in the processes of naturally democratizing and becoming capitalistic, therefore they are being exposed to American culture
Many tourist visit Vietnam and I guess that since their opinion of us is high we aren't being shitty guests?