I think it has a lot to do with their beliefs
After more than 3 years of living here I can honestly say that I don't really know what their religion is. There are a lot of Christians here for an Asian country but the most widely followed "religion" seems to have no deity and is just a series of things that are done in order to bring good luck. I should ask someone some day because it is intriguing.
I have never met or even heard of anyone that has anything against Americans here but they might exist in the North. Da Nang was kind of in the middle of all the conflict during the war and I have met a guy who owns a bar that was a little kid when the Americans occupied this area. He has a lot of photos of him with US soldiers and has even met with some of them later in life. He had to hide his book of photos for many years and even now it is not out in the open in his shop. He doesn't go into detail as to why that is but I can imagine that he was afraid of being persecuted for being helpful to the enemy.
As it stands now the Vietnamese have a "beef" with the Chinese who invaded many times over the course of hundreds of years and also the French, who attempted to take over the country. Perhaps the lack of animosity towards USA is because they were never trying to take the country away, just trying to do something to hurt Russia. I dunno!
There are museums depicting the atrocities of the war but for the most part they are quite friendly towards Americans and even have a memorial of sorts for John McCain who was shot down and captured as a POW.
That is all very interesting! It seems a bit hard to pin down exactly why they feel the way that they feel. It seems like it is just the complex way that history played out.
It's also interesting what you said about their beef with the Chinese. This weekend I was visiting family and I have a cousin who is married to a Vietnamese woman. She had a bunch of family around who were randomly talking smack about Chinese people. I thought they were just kidding, but maybe there is some real animosity there.
I think that when a neighboring country has been attempting to conquer your country for several hundred years only to get pushed back over and over again you probably start to have a chip on your shoulder about said people. I don't know the full history but yeah, the Vietnamese have a very fragile relationship with their northern neighbor. This probably explains a lot of why they always keep the Americans close and allow regular shows of US Naval might in the region.