This is a speculative outsider's view but a few things likely contribute to the pro-American statistic. My musings are based off of living in Vietnam for the last two years with my wife's family.
1. Half of the country was on the side of the Americans in the American War. The people that were on the losing side had a bad time after the end of the war. The Vietnamese refugees who went to the US had a much better time. Almost everyone in South Vietnam has a friend or relative who moved to the US.
2. An unusually large percentage of the population is young. They were born after the war and have been consuming American movies and pop stars since they were kids.
3. The Vietnamese don't seem to hold grudges for very long. For whatever reason, they seem to be a lot less traumatized by the war than the US was. This is despite the fact that they lost millions of people in the war.
4. China is currently the only real existential threat to Vietnam. It has been periodically invading Vietnam for thousands of years. The only ally powerful enough to curb a belligerent China is the US.
The other thing I would say is that I think the current economic growth is more a consequence of changes in economic policy in the 80s than anything that Bill Clinton did - https://en.wikipedia.org/wiki/Doi_Moi
I do agree however with you that the current US policy towards Cuba is silly.