Do you think that the US was the big bad in the Vietnam War? Without them interfering in Vietnam, millions of innocent people would not have dead, Vietnam would not have been ruined, billions of dollar would not have been wasted. And for what? After 40 years, the communist Vietnam is one of the US most trustful trade partners.
I think it was the French who got there first. We didn't really interfere with the territory as long as they were in control of it.
World War II was apparently the game-changer in all of that, as our interference in Vietnam began by aiding resistance against Japanese aggression. France was our ally, but the French and other colonial powers were too spent and exhausted from the war to be able to keep control of their empires.
This left a power vacuum in the world, with large parts of the globe resentful over what had been done to their countries and people. But by the same token, the US felt it had the obligation to contain what it perceived as Soviet aggression in various parts of the world which seemed susceptible to communist agitation. There was also the rather dubious "domino theory" that if we didn't stand up to the communists in Vietnam, the rest of Southeast Asia would turn communist.
It was all part of the overall policy of containment while trying to avoid direct all-out warfare with the Soviet Union. Anywhere in the world where there might be communists, we took it on as our own personal crusade to fight against them in any way we can, covertly or overtly, whether directly or by proxy. We saw ourselves as white knights fighting the evil empire (we still do). In some countries, it was relatively easy - simply a matter of bribing the right generals and a few key politicians. In other places, it wasn't so easy.
From a certain standpoint, Vietnam was just one part of the overall Cold War, which ultimately ended when communism fell in Eastern Europe and the Soviet Union was dissolved. The Sino-Soviet split was also something that the US was able to gain from, as our relationship with Communist China improved. Towards the end of the Cold War, the Russians were more worried about the Chinese than they were of us.
I don't know if the US was necessarily the "big bad" in Vietnam or other areas of the world. We've certainly thrown our weight around, although I think a lot of Americans see us as being the "big good."
As for relations with Vietnam today, I haven't followed things that closely, but it seems to be working for now. The world situation has changed. I don't think they're seen as any kind of regional threat, not like North Korea.