I think it has been covered before but there are a few problems with talking about this on both the Left and the Right (and by the right I mean neo-conservatives):
1. Johnson is a sacred-cow for the Left. His Great Society extended New Deal nonsense to an unsustainable point, but the Left cannot admit that the man was a failure. Doing so would undermine their admiration for the “butter” part of “guns and butter.” If Johnson was willing to violate his oath in Vietnam, why not in the United States?
2. Neo-cons cannot admit that the war in Vietnam was built on a lie. Doing so would taint the War in Iraq, also built on a lie.
The Vietnam War was an outgrowth of post-World War II foreign policy goals, namely Truman’s policy of containment and the fear of the domino theory in Southeast Asia. The French were booted out in 1954 and the United States filled the vacuum in an attempt to stop communist aggression in Asia. We supported a thug in Ngo Dinh Diem but he was our thug until the Kennedy administration had him assassinated in 1963 and replaced with another military thug again to stop the communist threat from the North (Ho Chi Minh).
I don’t think there are any other motives for American involvement in Vietnam other than the Cold War. It was seen as a place to halt the global threat of communism. The Gulf of Tonkin incident was the way to get the Congress to authorize force and to sell it to the American people, a la weapons of mass destruction. It had been reluctant to do so before.