There has been great discussion among historians and biographers of John F. Kennedy as to whether he would have committed the United States to the Vietnam War to the same degree that Johnson did, and there are no sure answers.
Part of our responsibility to help Vietnam was dictated by a treaty we supported--the South East Asian Treaty Organization--which required us to assist South Vietnam if called upon to do so. During Kennedy's presidency, he supplied advisers to the South Vietnamese, backed up by air and artillery assets, but not the level of troops we saw later in the war (1966-1970). But because Kennedy and his advisers understood that South Vietnam's leadership was tenuous at best and very corrupt, it is reasonable to assume that Kennedy would have been extremely cautious in deciding to commit US troops on a large scale. Several of his advisers, for example, would probably have advocated a very limited engagement of US troops, and one of Kennedy's leadership styles was to listen to everybody carefully, ask hundreds of questions, and then make a decision according to his own view.
During WWII, while he was stationed in the South Pacific, Kennedy wrote a letter to a good friend in which he said that one day he would like to be in a position to stop a war. Also, because his experience in WWII caused massive and debilitating physical problems for him, he would have been acutely aware of the cost of military intervention on any scale, and this awareness would, in turn, probably contribute to his reluctance to get into even a "limited" war like Vietnam.
Anyone who believes wholeheartedly that Kennedy would have avoided the Vietnam War entirely or, conversely, would have committed the US completely is engaging in revisionist history. There is evidence to suggest, however, that he would have engaged much more cautiously and on a smaller scale than his successor did.
It is really difficult to gauge the effectiveness of President Kennedy's leadership style in reference to Vietnam because there were more moving parts than simple leadership presence. Kennedy's style of leadership was to gauge as many opinions as possible from experts on a given topic. In the issue of Vietnam, Kennedy did not present himself as one who acted unilaterally, without the advice of advisers and counsel. This leadership style is, by nature, complex and intricate because it takes into account other individuals' point of view. This addes to the layers of difficulty in Vietnam, a situation in which nearly every learned individual had a diametrically opposed point of view. Consider the reports given to the President by the head of the Defense Department, General Krulak, and the State Department, Joseph Mendenhall. The former indicated that the war could be won with a full out military commitment, while the latter suggested that the political damage is impacting the United States by generating negativity towards the nation. In this, President Kennedy openly wondered if both men "visited the same country." This reflects how different opinions on the issue of Vietnam made clarity in policy fundamentally difficult. Adding to this was Kennedy's leadership style that sought to pull divergent opinions to find consensus. In a situation where consensus was difficult to establish, this leadership style compounded an already challenging issue.