JFK was not a peacenik. In his inauguration, he pledged to "pay any price, bear any burden, meet any hardship, support any friend, oppose any foe" in the Cold War.
He increased the number of US forces in Vietnam from a few hundred to 16000.
The real question is what would've happened if his brother RFK wasn't assassinated in June 1968 ?
RFK opposed the war. He was very popular amongst African-Americans and young people, and would've drawn upon the memory of JFK. He undoubtedly would've beaten Lyndon Johnson in the 1968 Democratic Primaries, and could've beaten Richard Nixon in the general election of 1968 unlike establishment pick Hubert Humphrey.
Then you may never have had a Nixon Presidency or Watergate, or the Vietnam War senselessly extended by a few years when the Nixon team knew the war was unwinnable.