Why would the Democrats nominate the losing VP nominee only four years later? I think it's more likely they nominate Humphrey for President in '64.
Could Humphrey get the nomination in 1964 though? The South is going to be dead set against him (with Wallace all but certain to make a third party run if they nominate Humphrey), and the northern party bosses aren't going to want to run a candidate that would split the party, so I could easily see the Democratic convention in 1964 deadlocking with Humphrey being unacceptable to the South and Johnson to the north, so the Democrats end up having to nominate a less prominent, but also less controversial compromise candidate. (Maybe someone like Senator Mike Mansfield of Montana who was a western liberal but also a long standing political ally of Russell and Johnson.)
At any rate, JFK had already lost and the party bosses may well block him at the '68 convention. Vice-President Lodge may end up as the GOP nominee, though Rockefeller or Reagan would have a shot.
Would Reagan even be on the political radar ITTL? No Goldwater campaign in 1964 probably means Reagan doesn't give "the speech" which got him so much attention in Republican circles that year. Furthermore, even if Reagan does still end up running for Governor, 1966 is likely to be a much less friendly year for Republican candidates ITTL as instead of being the midterm election in the sixth year of Democrat control of the presidency, it would be a midterm election in the 14th year of GOP control of the presidency, and thus the electorate is all but certain to swing hard against the Republicans that year.
(And as an aside if Pat Brown does win reelection in 1966 then he might very well be in the running for the Democratic nomination in 1968 himself as he would have a pretty impressive resume with 10 years as the Governor of California. Of course the "Catholics can't win" issue would dog him just as it would JFK or RFK.)
At any rate I would expect the GOP race in 1968 to be between Goldwater, Rockefeller, and Lodge with Lodge (presumably backed by Nixon) probably getting the nomination as the compromise candidate.
I don't see why Nixon wouldn't escalate the number of advisors as Kennedy did, and when it comes to the point when he has to decide between war and the collapse of South Vietnam then there's no reason to believe that Nixon wouldn't make the same mistake that Johnson did. After all, Nixon escalated the Vietnam War from 1969 to '70 IOTL.
If Nixon invaded Cuba and has already had to fight a nasty guerilla war there, he might be reluctant to commit ground troops to Vietnam.