Haig is an interesting character. He really is one of the first of the neo-cons. It was Alexander Haig who sped TOW and air to air missiles to Israel during the Yom Kippur War (October 1973) when Nixon and Kissinger dithered, intending that Israel be partially defeated and forced to give back all territory taken in 1967. Israel, however was in danger of being overrun by Syrian and Egyptian forces, according to Mark Aarons & John Loftus (The Secret War Against The Jews) and likely would have had to use it's nuclear weapons. Haig sent American weapons to Israel without authorization from Nixon and managed to keep his job after doing so.
This is an act that would not have been quickly forgotten by Israel's supporters, either amongst Jewish Democrats or conservative Christian Republicans, either had Nixon nominated him as Vice President or in either subsequent election (1976 or 1980).
Interestingly, Colodny (Silent Coup) claims that Haig was one of Bob Woodward's sources (though we know Haig was not "Deep Throat", Feist was). If so, Haig was not the Nixon man people thought he was, and Washington insiders would know this.
Between Haig's independence from Nixon and his exhibition of that independence exhibited by his action on Israel, I think, with help by Israel's supporters in Congress,
And South Vietnam and South Korea's supporters in Congress.
Haig would have gotten confirmed. I also think that Haig would have been re-elected.
At that point, the butterflies start to fly.
And that's where our ATL can get very interesting.
For one thing, Haig would not let South Vietnam go down without a fight. If Congress would not allow any US funds or US personnel for airstrikes against North Vietnam, I suspect that Haig would be perfectly capable of Iran-Contra type shenaigans to work around that prohibition. Perhaps by selling or leasing some aircraft to the Philippines, getting some right wing Japanese industrialists like Ryochoi Sasagawa to bankroll the airstrikes and bringing in Israeli pilots on loan to fly the missions. The kind of thing that we got used to in the 80s, 90s and 2000s but were unheard of in the 70s.
And when Congress hears about it, DARING them to impeach him--which would endear him to the people we call ITTL "Reagan Democrats" and we would call ATTL "Haigocrats".
So South Vietnam probably survives 1975 despite Congressional Democrats hoping that it dosen't. And as close as the 1976 election was ITTL with Ford, the more charismatic (and more ruthless) Haig probably wins it, perhaps with some of the kind of vote suppression we have become used to in our time ITTL.
I shouldn't spoil it any further, but I doubt if there will be a hostage crisis during a Haig Administration because I see Haig, for better or worse, backing the Shah to the hilt even over Saudi Arabia and even if US troops are necessary to keep him in power. There is sure to be a different crisis when the Shah finally dies of cancer, but not this kind of humiliation.
Haig would not be able to run for a second term in 1980 under the 22nd Amendment. Nixon resigned too soon. Only Vice Presidents who take over more than halfway into their President's term are permitted to run for a second term. So the 1980 nominaton would belong to whoever Haig made his VP most likely.