The official US line was that the US wanted to stop communism. But if that was a reason, it was far down the list.

Vietnam was part of what the US called a "Grand Area." It also included Indonesia and the rest of SE Asia. The purpose of the "Grand Area" was to be a cheap source of raw materials and labor after WWII, to support growth in the US AND Japan, because the US wanted to rebuild Japan as a strong capitalist country to help keep the USSR and later China in check.

For each of these countries, the ideal situation was for them to become independent but with a government compliant to US needs. The US wanted to end *European* colonialism in the region because that would allow US capital to become more dominant. Compliance was needed to enable this, and was much more important than fighting communism.

The issue was that in almost all of these countries, the remaining anti-Japan resistance from WWII was run by communists. Communists like Ho Chi Minh who enthusiastically supported the US during WWII, and who was now labeled an enemy in Vietnam by the US because he would not be compliant with US capital needs.

The reason why there was a North and South Vietnam was that the US wouldn't allow a nationwide election to take place because if it did, the communists would win and the US would be shut out.

It's a difference that isn't talked about from the US perspective. You should find that Newsweek article, because it touches on it. here's an excerpt followed by the link:

"All this history pretty much evaporates at the point in the Burns-Novick series when the U.S. government begins justifying its growing military intervention in Vietnam, first under President Kennedy, and then Lyndon Johnson. Washington policymakers (and the first wave of the American reporters in South Vietnam) redefine the war simplistically as a fight for freedom. One of those reporters, the late David Halberstam of The New York Times, would later say that, “you really should have had a third paragraph in each story which should have said, ‘All of this is shit and none of this means anything because we are in the same footsteps as the French and we are prisoners of their experience.’”