This subject is a source of great controversy, as there is absolutely no definitive way of knowing if, had he lived, he would have followed the same course Lyndon Johnson did in massively escalating US involvement in Southeast Asia.
Kennedy was a brilliant man who, during his short time in office, was already becoming an iconic figure, especially to those of us who were growing up at the time. His assassination then made him a martyr and a near-mythic personage, perceived by much of the country as a heroic symbol of liberalism. Yet Kennedy had endorsed the very ideas which, just a few years after his death, were anathema to the anti-war movement and began to be questioned even by conservative, hawkish Americans. He clearly believed in the "domino theory" that if the Communists took over one country in Asia, then another country would soon fall to them, and another, and another. This was why he had ordered combat troops into Vietnam in the first place. Would he have been wise enough to know that the situation would become a quagmire, and that the Vietcong and North Vietnamese would fight on and on, regardless of how much bombing we did and how many of them were killed? Based on his own military experience in the Pacific during World War II, he probably would have had a greater understanding than LBJ had of the kind of warfare being waged in Asia in the 1960's as well.
Those who served in the Asian theater in the Second World War knew, from direct combat experience, that the Japanese would have defended their homeland to the last man had the atomic bombs not been used against them. US casualties in an invasion of the Japanese mainland would have been enormous. The attitude of the North Vietnamese twenty years later was not unlike that of the Japanese. It's quite likely, in my opinion, that Kennedy would have realized this and would not have wanted to prosecute the war as Johnson did, escalating it beyond any proportions that made sense, especially given the eventual massive opposition by the US public. But this assumption, of course, about what Kennedy, or any politician of the past, "would have done" in a given situation is merely hypothetical. And there is not enough definitive evidence that JFK had any plans for de-escalation. The assassination, though the enormous tragedy it was, absolved him from having to make those decisions which then fell to Johnson, and later, to Richard Nixon.