We embargoed Vietnam until 1994, IMO we changed strategy and improved our relationship with them primarily as a hedge against China (you know this is the case because Joe Biden recently said that it's not lmao). In South Korea and Japan we hedge against China by forcing a bunch of military bases onto them, but since Vietnam isn't a vassal state we had to take a different approach, and bear in mind that 1994 was the Clinton administration, when our country's global credibility wasn't completely in the toilet as it is now, so we were able to make deals and actually build something, in this case US-Vietnamese relations.

Vietnam had sided with the USSR during the Sino-Soviet split for a lot of reasons, and without the USSR they were looking for a new non-Chinese friend and major trading partner, a role which we gladly took on. Vietnamese people generally have positive interactions with Americans, since most Americans who visit are older tourists or military members who spend a lot of money but don't actually live in the country long enough to commit a bunch of crimes (not that crimes don't happen, it's just fewer than what SK and JP have to deal with). Combine that with the Vietnamese-Americans who maintain relations with their families in the mother country (the fact that many of them fled the South because they were afraid of the Communists redistributing their land is water under the bridge by now), and you have a recipe for a lot of soft power to build up in a very short amount of time.

Sharing borders with China, they have culturally-held reservations /anger /fear /disgruntledness about China's influence and effects reaching within their lands, and believes China had invaded them or ruled over them unfairly at various points from ancient times to the Vietnamese Communist era, etc etc. All the typical love-and-hate between lands that were side by side for thousands of years with endless cycles of good times and bad. Relationship status: it's complicated.

So Vietnam geopolitically aims to find other great powers—USA in this case, USSR in the past, etc—to counter balance China. Simultaneously, they have been part of the sinosphere for centuries and does know the need and the how of working WITH China too.

The government is more pragmatic but the population adores America and worships them on levels close to the Philippines. Soft power goes a long way.

I feel a bit embarrassed to tell personal anecdotes, but here I go...

I visited Hanoi in 2005. Most people very nice, but I did feel random animosity from older men (were they vets?). Occasionally they would insult me (I assume, I speak zero Vietnamese). I'd get a the ground near my feet spat at. It was fine, but I felt the vibe.

I went last year, and felt none of it. I can't get over the level of interest and respect that I got, a very mediocre middle aged white dude. Multiple times I had people far beyond my level of cool trying to be my best mate. Also I got beaten in chess by 7 different people who were super humble about their ability.

Edit: I forgot the add, people complained about China a lot.

China was at war with Vietnam more recently than the US, and they have a number of border disputes.

America is now out of their space and is a trading partner whereas they share a border with China and have South China Sea disputes. China and America are rivals so it makes some sort of sense for the Vietnamese to pick the one not too close to annoy the other.