I think @kritiper has it right. The US seems to have a good justification for their action in Iraq: because they are the homeland of some super stereotyped extreme terrorists, and there have been reports of them causing havoc everywhere in the world. Meanwhile no Vietnamese had ever caused direct harm to the US. All we did wrong was embracing communism. And seems like the 1960s America had some hipsters who were against capitalism, and some people who genuinely thought we didn’t do anything wrong to the US.
I should mention that I saw a handful of media (movies, books, games) from/set in the 70s, and when the Vietnam war is mentioned, most people just acted indifferent, very similarly to the attitude you see in the current Iraq thing. The most extreme reaction was in a movie I watched long ago don’t remember the name right now, have to look it up. In the movie, a character mentioned that his brother fought in Vietnam to free the “country of Saigon”. So it doesn’t seem like everyone in the US was against the Vietnam war.