From what I've read, Humphrey's hawkishness was also based on his own personal conviction. In a 1967 address to the ADA he said "from Munich till today..." which hardly sounds like a faintheart mouthing lines he didn't really believe. IIRC he never recanted his Vietnam stance from those years, even in the 1970s.
I'm speculating about him remaining as Senator Humphrey after 1964, not VP Humphrey. As VP he was wedded to his administration's Vietnam War... Sort of like Bobby was wedded to his brother's Vietnam policy (though luckily he was able to get away with "that was all
before the escalation" as a defence.)
I'd go as far as to say Humphrey's support for Vietnam would track that of the man who replaced him in his seat, Fritz Mondale. That is, he'd be in favour of the war at first, he wouldn't be as sceptical as the likes of Morse and Fullbright, but he'd join the rush for the exit once it became obvious that the administration was lying about 'the light at the end of the tunnel'.
I can't see him being a deadender, even if his progressive friend Paul Douglas demonstrated it was possible for a liberal to be a deadender on Vietnam.
HHH wanted to be president, and if he had never served as VP in a war administration he'd quickly come to the conclusion that he had to get out in front of the issue as a cleanskin. Hence my belief that a never-served-as-VP Humphrey is the only New Politics presidential candidate who could possibly win in 1972.
Though I gather you don't believe RFK was ever anti-war, in which case I don't think we'll be able to agree on the definition of anti-war-Democrat after Tet (I do agree with the contention that Bobby Kennedy wasn't a very good New Politics candidate.)
If Humphrey doesn't run in '68 then Scoop Jackson runs. Someone has to run as the LBJ proxy simply because there's no way Johnson lets Kennedy skate to the nomination. Whether Jackson runs in the primaries himself or uses the proxy strategy employed by HHH IOTL is certainly debatable.
If VP Humphrey doesn't run then surely LBJ will push for John Connally.
I don't know enough about Jackson to know whether he thought he had the political capital to run against an upstart Kennedy campaign and/or Connally and/or the threat of LBJ deciding to break his promise to resign by jumping into the convention. We're talking about political calculations that go beyond whatever ideological motives these guys had. I actually take it for granted that a HHH who'd stayed in the senate wouldn't run in '68, seeing as how there is an incumbent Johnson VP likely to seek the nomination and there's Kennedy as well.
This ideological laundry sort of also applies to the Kennedys...