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.