This is a hotly contested topic, which is exactly why I avoided touching it in the first post.So.....if we look at Vietnam.....what would Kennedy had plausibly done? :/
There's entire Kennedy assassination conspiracy theories claiming he was shot because he wouldn't have authorized a build-up in Vietnam. Popular legend often tries to tell us that Kennedy was too intelligent to get us wrapped in some crazy war halfway across the country, for one reason or another, but historical fact is tricky on this. It's known Eisenhower warned Kennedy about Indochina, but Kennedy did authorize military advisors. Vietnam OTL was a colossal fuck-up no matter whether you blame McNamara, Westmoreland or LBJ.
Some theorize Kennedy would have fallen prey to the same problems that plagued Johnson - paranoia about America's reputation abroad, being fed distortions of the truth by advisors and supporters, and his own fervent anti-communism. History could have re-aligned similarly, possibly without the added benefits of the valuable Great Society legislation passed in the wake of his death. That's one theory.
There is also, of course, a viewpoint Kennedy would have kept the United States involvement in Vietnam partially, but would not have escalated the conflict to OTL levels, which seems to be the favored viewpoint on these boards, but I'm not sure if that extends academically. Keep around military advisors but let the Vietnamese handle it on their own. This is especially tricky because Diem, the leader of South Vietnam, died shortly before Kennedy, so he never worked with the post-Diem Vietnam extensively enough to study.
It's important to remember that while hindsight tells us Vietnam was bad, pretty much anyone 'important' at the time was at least initially for the war, until they came out against it - even the earliest detracters said when the Gulf of Tokin resolution passed, none of them expected Johnson to get stuck over there.