Interesting question.
I don’t think the Great Society programs would have been passed because Kennedy wasn’t that interested in domestic policy. Plus, LBJ knew how to work Congress to get his bills passed. Kennedy not so much.
What Kennedy would have done in Vietnam is a real question mark to me. If I had to guess, I would say that he wouldn’t have escalated the war like Johnson did but I don’t think he would have withdrawn U.S. military forces from the country either.
And I do think the 1964 election would have been a lot closer.
I think Vietnam would have been much more Special Forces and airpower and much less or no main line combat forces.
Johnson’s approach was “Step aside little yellow brother while we come in and roll back the Commie tide!”
The South always had the capacity to defend themselves. We pushed that to the very back burner in 1965.