it was indeed in south vietnam. the u.s. was trying to stop the spread of communism. when north vietnam (which is communist) tried to take over the south. The U.S. felt that capitalism and democracy would be threatened by this so they sent "aid" to vietnam. not necessarily to defend the south vietnamese. but to try and deal a blow to communism.
The french never called upon the help of the u.s. during their indochina wars. As lonesniper stated. It was some time after the indochina wars that the U.S. got involved. and the french has no part in anything of that second indochina conflict involving the u.s.