At it's core the Vietnam conflict was a struggle by the Nationist Vietnamese to gain their own real live country to call their own.
Near the end, by the time the U.S. was forced to become aware of it, the North Vietnamese were Communists on the surface, but nationalists at their core, the Communism was just a nation uniting ethos.
SO, your challenge, should you choose to accept it is to prevent the 12 years that we spent pouring blood nd toil and sweat and tears down a hole to essentially no real purpous by preventing AMerican Entry into the Vietnamese Civil war.
AAAAAAAAND, GO!