Almost impossible as in the eyes of LBJ, it was the Kennedy's who got the U.S. heavily engaged in the war in the first place.
Also you would have to have no Gulf of Tonkin Incident. Not the famous, infamous one where the North Vietnamese didn't actually attack at all, but the one two days earlier where the North Vietnamese torpedo boats did indeed attack the U.S.S. Maddox (admitted to by the North Vietnamese General Giap in 1995).