As we're getting to the 50-year mark past the American involvement in Vietnam, things are progressing in ways not anticipated at the time. Vietnam, like several other Asian countries, is embracing capitalism and entrepreneurs. Old enemies hug and pose for photographs.
This seems to be a pattern, at least among American conflicts (I have not studied others very much). As the combatants grow into old age, it's not worth carrying a grudge around. Things that were worth dying and killing for 50 years ago are still important, but they're fights for young people. It becomes easier to see things from the other side.
I'd argue that by simply having a huge presence of Americans in Vietnam for so many years, the two countries have become joined for the next century or so, much the same as having kids in a marriage joins the two partners together even if they can't stand one another. I also note that things may end up turning out better for Vietnam in terms of economic development than if South Vietnam had won the war -- which would have only institutionalized worse corruption than they currently have.
This is all speculative, of course, but I seriously doubt anybody in the mid-70s would have guessed that this is where we'd all be 40 years later. It's been an interesting journey for both countries.