I don't see RFK running in 68' if his brother served a second term. Maybe a few years later RFK would run for President, but it would've been difficult for him politically to run while his brother was still in the White House.
He leaves the AG position, say after three years?, to run. If not him then who does JFK support in '68? Clearly not LBJ but then who else? Trying to think of an alternative choice that JFK would support.
Re successful second JFK term: I think the idea that JFK would/could have simply pulled out of Vietnam after being re-elected ignores the consequences of such an act. The South falls - in a year? two? - and then the entire region goes up in flames. The people in the South flee to Cambodia, that causes internal problems for them that spill over into the entire region. The Pathet Lao and Khmer Rouge are emboldened. Remember, Khrushchev - who showed little interest in supporting Hanoi; he thought it would benefit Mao and China - was replaced in 1964. The new leadership was much more supportive of Hanoi. So an aggressive, Soviet-aligned Hanoi is in power. Can he just leave?
JFK would have had a mess to deal with; just as LBJ did. Trying to muddle through with half-measures, as LBJ tried (bombing and pause, bombing and pause, restricted bombing zones), didn't work. But a full out war, no bombing restrictions, would be a hard sell. Would JFK recognize this early on? It's all a guess.
