Vietnam is seen as a disaster is because America LOST.
We didn't lose so much as quit. I know I'm in the small minority with this view and it probably stems from internal biases of mine, but the military didn't lose. They won all the major battles, had a silly K:D ratio, and could go anywhere they wanted to at any time. The other side lost all the major battles, lost an insane amount of dudes and were extremely limited as to their mobility. The American public lost their will to fight because it was on TV, every night, in their face. I'm in no way saying this is a bad thing, this is a good thing. We shouldn't have been there in the first place.
But we didn't lose, we quit while we were way ahead.