I think that Oliver Stone’s belief in the Vietnam situation being the reason that JFK was assassinated is nonsense. I don’t believe it whatsoever. My point is simply that I believe that JFK would not have sent U.S. combat troops to Vietnam. The plan at the time of his death was to reduce the number of U.S. military advisers back to the level it was before he took office. I think JFK probably would have been willing to negotiate a settlement after the 1964 elections. Sadly, LBJ had a different viewpoint. He took the situation personally. And he was willing to send hundreds of thousands of soldiers to fight so that HE wouldn’t be the first U.S. President to loose a war.
The plan as I read it was to withdraw forces as the South was able - through our training, assistance, aid et cetera - to take on more of the battle. It wasn't simply to withdraw regardless of the situation on the ground. Two things were to happen simultaneously: we withdraw as they step up. So what would JFK do it the South wasn't able to accomplish that? That's what happened. That's what LBJ faced; and what JFK would have to face. That's why we had to take on more of the effort or leave. Nixon tried to reverse that but it was too late.
As to JFK and a settlement: What's the evidence that Hanoi wanted one? Or Moscow either? After 1964 the Soviets fundamentally changed their approach to SE Asia and, after Khrushchev's removal (he was reluctant to get involved because he believed it would help Mao), began providing massive military and economic support to the North.
JFK was able to settle - for a time - the Laotian crisis without sending in troops because Khrushchev agreed to one. The historian and conspiracy believer John Newman argues that JFK would have dealt with Vietnam like he did with Laos. That is, not send in ground troops. But that ignores the key fact that Soviet policy towards Vietnam was completely different after Khrushchev then it was when he was in charge.
As to LBJ: he uncritically listened to his advisers, the "Best and Brightest", who gave him bad advice and didn't like disagreement among his people. As you pointed out, JFK was much more willing to challenge his advisers, especially the Pentagon and liked hearing dissenting views; so while LBJ went along with their advice, JFK was probably going to reject it.
Anyway: again, what JFK wanted to do is not the same thing as what he was able to do. I don't think LBJ wanted to send in troops either. There are records of conversations he had with people where he said he didn't know what to do. We had to get out but how? What about the consequences?