JFK had been gone for a while before the Gulf of Tonkin events happened. LBJ’s mindset regarding Vietnam was different than JFK’s mindset. No one can say with certainty what JFK might have done regarding Vietnam if he had not been assassinated. But he never strayed from his opinion that the conflict was one that the Vietnamese had to resolve. He believed that the U.S.’s role should be to assist the South Vietnamese, not to fight the war for them with American combat forces. LBJ, on the other hand, let his ego and insecurities get in the way of good judgment. He said that “HE wasn’t going to be the first U.S. President to loose a war.” The conflict was Vietnam’s war until LBJ decided to Americanize it by sending U.S. combat forces. Once Nixon became President, he began to re-Vietnamize the war by reducing U.S. forces and training Vietnamese forces to take the place of them. This, by the way, was exactly what JFK was doing when he was assassinated.
Well, we've gone round-and-round on JFK and Vietnam. JFK's top advisers all wrote books about their experiences. McNamara, Bundy, Rusk. Even RFK talked about it. All said that there were no plans, none, to simply leave the conflict. That's what they all said. In November of 1963 they still thought the war was winnable. The Pentagon Papers, the top secret history of the war, also said the same thing.
At what point does a JFK recognize that it wasn't? Who knows? What does he do? Leave? Or try to find a way out? As the people above said: that idea wasn't even considered since at that time they thought it was still possible to keep the South from being overrun. If JFK decides to leave, the South falls, in a year? six months?, and the entire region is likely thrown into disarray. Refugees pour out of the South (as they did in 1975) and destabilize Laos and Cambodia. Those governments fall to the communists. So all of SE Asia "dominoes" into communist control. Can JFK allow that to happen? Politically? He's not up for re-election but he wants, probably, RFK to follow him. Or a Democrat. Can he throw his party overboard? What happens in the rest of the world? Again, we're all guessing about what he'd do if he decided it was unwinnable without putting ground troops there.
I insist on one thing though: If you asked JFK on November 22, 1963 what he was going to do if the South couldn't defend itself, it his policy of building a self-sustaining government after removing Diem failed, what would you do? He would say, "I don't know."
In any case, Stone's thesis that JFK was going to leave and it was that, in large part, that he was killed is a flat out wrong. I won't say lie because I'm sure Stone believes it. But he's a ridiculously misinformed person.