Despite what the USA did in its invasion of Vietnam, not because of it.

Vietnamese are trying to not forget their history: https://en.wikipedia.org/wiki/War_Remnants_Museum

(I'm not sure how many Vietnamese actually love USA, vs how many don't... I just want to remind that different people in the same society might hold different opinions, and the sentiment is certainly not monolithic)

I never really looked into it, but it looks like the vast majority of Vietnamese were born after the war so US culture and trade are way more important contributors to opinion. Vietnamese are some of the most pro-US people in the world.

Vietnam had such massive population growth that there are very few people who even remember the war. On the other hand China was pretty much always ingrained into their “national consciousness” as a permanent massive threat.