What was the name given to the idea that if vietnam fell to communism, the rest of southeast asia would become communist?

It was called Domino Theory.  The U.S. believed that if Vietnam fell to the Communists, other countries in Southeast Asia would follow. They feared that if that were to come true, then it would be a matter of time before all countries in the region would fall to Communism.  That is why they set up large military presence in the region to deter the spread by supporting some countries in the area.