I resemble that very much!
I wasn't much focused on what happens when Humphrey and Wallace and the other Southerners shake hands on the Congress deal. Indeed, Humphrey would be wobbly being the first President since John Quincy Adams to be chosen via the Congressional process, but if the deal involves stuff that gratifies enough Southerners, it could well leave him in a pretty solid position come 1972.
Re Vietnam, Johnson had been negotiating a peace treaty with Hanoi and Moscow for some time before the election. Nixon infamously (it was secret at the time, but has become well known since) and criminally (but LBJ knew via illegal channels and so could do nothing) torpedoed the treaty by lining up the South Vietnamese leadership to refuse to consider it. But that was contingent on Nixon's promises to give the South (or anyway, its crony leaders) a better deal as President, and if Nixon goes down for the count in '68 I figure he will fade out of the picture. Well maybe not, but anyway the South Vietnamese leaders will quickly understand their dependency on the patronage of Washington as it is, and they can't hold out until 1973. If Humphrey can pick up in a few months where Johnson left off, a peace treaty might be a done deal before 1969 is out.
The treaty LBJ was pushing is said by every source I have seen to be essentially identical to the one Nixon got after being reelected in 1972 OTL, so while that is hardly reassuring as to how good a protection of the South Vietnamese people it might be, it should be viable in the most cynical sense anyway for domestic US politics purposes. In fact, if the treaty would provide for the USA to maintain some tripwire military presence, the South ought to be protected quite well from the manner in which it fell OTL--whether that secures it from the prospect of falling to civil war is another question, assuming the US forces recuse themselves from being involved directly. However, many people assert that the Tet offensive exhausted the South Vietnamese National Liberation Front forces, and that willy nilly, a commitment to the South Vietnamese state was in fact on the rise, and given effective shielding from overt Northern invasion which is what it took to topple the Saigon regime in 1974, due to Northern forces not daring to advance openly in tanks and armored columns into a nation where US forces even in token numbers stand by to trigger the fast deployment of more of them from say the Philippines or Thailand or Taiwan (no Nixon Goes to China if Nixon is not elected!) and involvement of the US military in the kind of war our forces were trained to fight and win in the honorable and politically defensible cause of defending a sovereign ally from a plain and simple military invasion, I suppose the survival of the South has a fair shot. Anyway a more cynical form of the treaty has the effect of washing US hands and getting our boys out. The difference between Humphrey and Nixon, or any Democrat probably and Nixon, would be accomplishing this four years earlier, because it is also well known that Nixon would boast of using the Vietnam War as an issue to get reelected and had no intention of settling it any early than his second inauguration. Any Democrat, including by late 1968 LBJ himself, had every intention of ending it ASAP and would have I think. And would be able to, albeit perhaps not without dooming South Vietnam. But getting a form of the treaty allowing the South to be protected by American tripwire forces in small numbers might have been a far easier political sell in in the USA in 1969, because while the popular narrative goes that a bunch of hippie peaceniks brought down LBJ, I think it should be clear that the subsequent four years of deception after deception regarding Southeast Asia wore down the credibility of the US government in general and on Southeast Asian issues in particular to make general skepticism far more widespread by 1973 than it ever was in 1969. A Congress that could be sold on ongoing US presence on limited and secured bases in South Vietnam coupled with contingency plans to swoop in with adequate force to stop a maximal North Vietnamese invasion, but with no American GIs roaming the jungles looking for VC villages to set afire, and maintainence of civil order in the South entirely the Saigon government's problem (on paper...in truth surely a lot of CIA activity would be shoring them up and assisting but none of them are draftees brought home in body bags; on paper any dying doing that would not be listed as having been anywhere in southeast Asia at all probably) seems far more likely in 1969 than in 1973, by which time the mood of the votes was, get the hell out, screw the South!
So Biafra might be worth saving, but consider the vastly greater impact of the entire Vietnam war being over before 1970, with the US having had a credible win of sorts, and no escalation into Cambodia. Doable I think!
I do think the stagflation of the 1970s was pretty much sure to happen; it used to be fashionable to blame Nixon for that too but you all can credit my Marxist economics for disbelieving this could have been substantially the case. A little bit, yes, probably, but it was coming like a freight train--or rather the irreversible sinking of said freight train's track into a very sticky swamp--for fundamental reasons I believe. And no liberal power did well in trying to deal with it, but perhaps OTL the misleadership of Nixon and his preoccupation with paying for his useful 'Nam habit (not to mention the sweet rich pork this brought his corporate patrons) closed off some useful avenues, that if not tried in America that some European government might have greater freedom to try, and thus show a better way out of the mire.
Similarly, while it is hardly a slam dunk, I think Humphrey cutting a deal with Southern leaders might actually hold out hope for more effective civil rights outcomes, especially in the South. (Anyone who thinks drastic change in "racial" relations was mainly something only Southerners needed to embrace fails badly to understand the modern USA, or at any time really).
On the whole then, I think Humphrey winning via Congress with a hard deal with southern leaders could result in quite a nice improvement on OTL. It might not be able to fix the incubus of stagflation fixing to land on the world's back, but even without that quite a few nice wins might leave us better off.
And save tremendous numbers of lives in Southeast Asia at least, and possibly in lots of other trouble spots around the world.
I think Humphrey would be well place to win in 1976, the biggest problem by then being that it would then be 16 years of Democratic presidents; by then Republicans might seem shoo-ins no matter how well Humphrey is seen to be doing the job, just because it is the Republican turn. Also, they would not have their brand defined by Nixon nor tarnished by Watergate.