Both men were politicians. In those times, Democrats had to take a tough line on Communism publicly in order to defend against the "soft on Communism" label.
Given JFK's somewhat sympathetic view of anti-Colonial movements in his time combined with his well documented resistance to the Pentagon's efforts to escalate US involvement in Vietnam during his first term, it remains plausible that he wouldn't have made the same mistakes as LBJ. I don't think we can assume that he would've taken the same exact path.
But I'm also aware of the fact that it was politically risky for him to be viewed as "losing Vietnam to the Commies" and in the end, politics might have won.
It's very similar to the logic that has kept the US in Afghanistan for almost 20 years.
Well, we're going around in circles.
As I've said (sorry for the repetition), the evidence that JFK was sincere in his beliefs is supported by the statements by the men around him - McNamara, Bundy, Rusk,
JFK RFK - and the documents. The Pentagon Papers document the internal discussions going on in the Administration as to what to do about Vietnam.
This isn't just JFK spouting off lines to protect himself from criticism about "losing Vietnam." They sincerely thought that a loss of the South would have ramifications for all of SE Asia and would danger American interests and security. They didn't view this as a internal anti-colonial effort by the North. They saw it through the perspective of the Cold War. And the evidence is they were worried about Chinese support for Hanoi more than Moscow's. And from what I've read, Moscow, both under Stalin and then Khrushchev, was reluctant to support Hanoi; they thought it was useless for their interests, that Mao's interests would be better served by a conflict there if they helped (the Sino/Soviet split was deep and real); and they were more concerned with other matters such as Berlin. That all changed after Khrushchev was removed.
The Pentagon Papers says this, something that I think accurately summarizes JFK's view:
"In the course of these policy debates [i.e., how to deal with Diem], several participants pursued the logical but painful conclusion that if the war could not be won with Diem, and if his removal would lead to political chaos and also jeopardize the war effort, then the war was probably unwinnable. If that were the case, the argument went, then the U.S. should really be facing a more basic decision of either an orderly disengagement from an irretrievable situation, or a major escalation of the U.S. involvement, including the use of U.S. combat troops. These prophetic minority voices were, however, raising an unpleasant prospect
that the [Kennedy] Administration was unprepared to face at that time. In hindsight, however, it is clear that this was one of the times in the history of our Vietnam involvement when we were making fundamental choices. The option to disengage honorably at that time now appears an attractively low-cost one. But for the Kennedy Administration the costs no doubt appeared much higher. In any event, it proved to be unwilling to accept the implications of predictions for a bleak future. The Administration hewed to the belief that if the US be but willing to exercise its power, it could ultimately have its way in world affairs.
Again: "The costs [for a withdrawal] no doubt appeared higher..... [And they] hewed to the belief that if the US be but willing to exercise its power, it could ultimately have its way in world affairs." Whether or not JFK himself believed in the "domino theory" or not the evidence is powerful, for me, that he simply didn't believe we could just walk away.
The entire section, well worth a read, on the Diem crisis and its aftermath is here:
https://nara-media-001.s3.amazonaws.com/arcmedia/research/pentagon-papers/Pentagon-Papers-Part-IV-B-5.pdf 
