After World War II, the main purpose of the United a States in regards to foreign affairs was the containment of Communism. After the French surrendered Vietnam as a territory, it became "threatened" by Communism, lead by Ho Chi Minh.
The Containment of Communism and protection of democracy was a driving factor regarding what happened with Vietnam.
The consequences of the failure in Vietnam led to it eventually falling to communism after concessions were made as well as Laos and Cambodia falling to communism. However, the other allies of the United States in that region did not fall to communism as well as the domino effect did not occur.