US involvement in Vietnam was based on complete misunderstanding. The US is not know for their cultural or historical intelligence to begin with, but McCarthyism had wiped out many China and Asia experts from the State Department.
US misunderstood the history of Vietnam and the goals of the belligerents. Vietcong also misunderstood the US.
Ho Chi Minh and Vietcong were nationalists first, communists second. US feared the domino effect that had no change of happening with Vietnam. Vietnam would not side with China for any reason and China was prepared to go to war against Russia if Russia got too much influence in Vietnam.