Vietnam has a very specific and valid grudge against China. Granted a grudge against the US would also be super valid. I think it falls along the lines of China should have been their ally so from them they felt betrayed. America just gave them what they expected from western imperialism. This is vibes based though so take it with multiple grains of salt

China probably feels pretty threatening to live next to. See: why many countries bordering Russia really really dislike Russia.