Without American boots on the ground, and without bombing of the North, the South was doomed. Kennedy would see that, and slowly withdraw.
Why would he see that?
LBJ and his advisers could quite clearly see that Vietnam was a fustercluck and couldn't see any happy ending - but defeat wasn't an option, so they held back as long as they could, hoping things would just get better, and then poured in men and treasure when it was clear South Vietnam couldn't hold with just advisers. No-one could see light at the end of the tunnel in Vietnam. There was no plan for getting out, only the hope that somehow something good would happen and an opportunity would present itself if they kept raising the stakes.
Seeing as this was the exact same choice that every US politician took between 1945 and 1973 and Kennedy had already chosen to greatly bolster the US forces in Vietnam, I don't see any reason to credit Kennedy with foresight greater than those around him. Indeed, based on his record, my bet is that a surviving Kennedy bungles Vietnam even worse than LBJ did.
A Nixon victory in 1960 might lead to South Vietnam falling early. I would rate it as unlikely though. Nixon in the 70s didn't loose Vietnam by choice - he secured what he thought was a victory and then, when the North Vietnamese broke their treaty with him and attacked South Vietnam again, accepted that he'd actually been defeated. And the Nixon of the 70s knew much more about the international situation. Likely his determination to not be the president who presided over the "first American defeat in war" would see his younger self getting sucked into Vietnam just like LBJ.
The best opportunity to avoid the Vietnam war is back in the 1940s, and have the US choose Ho over the French, which might have happened if the memos from the OSS people on the ground hadn't been lost in the bureaucratic jungle between them and the decision makers.
But... Let's say that Nixon DID win in 1960 and DID decide to pull the "advisers" out of South Vietnam... South Vietnam itself was deeply dysfunctional and I can't see it surviving more than 5 years without US troops. After that... I have no idea what happens next to be honest. The US visited utter ruin on the population and something like 10% of the population died during the war. Not to mention the mis-spent treasure of the Vietnamese and the damage inflicted to the environment and to existing infrastructure. Potentially, what it means is that instead of Vietnam having a gdp/capita 1/4th that of China, Vietnam equals China in its level of development.
That wouldn't be a small thing - TTL's united Vietnam could have a population of 107 million (as opposed to OTL's 96 million - I'm assuming here that a Vietnam without a war with the US would grow at the same 1.8% a year on average that OTL's Vietnam did) and a GDP of 800 billion-1 trillion US $. They'd be up there with South Korea in terms of being regional powers.
fasquardon