Others have mentioned that the data is misleading and the effects of time
But there's also the fact that they are communists. They look at the American people as separate from the ruling class.
Additionally they recognise the efforts of many Americans in stopping the war. And many have had positive experiences with individuals, both during and after the war.
None of this minimises the atrocities of the US state or the individual atrocities of US soldiers. But the Vietnamese people have risen from the war, and are used to making friends with old enemies like China.