To the OP:
The Million Dollar Man angle didn't come into fruition until after Wrestlemania 3.
Hulk beat Andre at WM3
Then MDM bought Andre's services to try and win him the belt on a Friday night Main Event match because MDM couldn't beat Hogan himself. This was when there were 2 Earl Hebners, which ended up being Earl's twin brother.
That's when the title became vacated because Andre won, with the twin, and he then gave the belt to DiBiasi.
Wrestlemania 3 had nothing to do with this because all this came afterwards. The DiBiasi stuff came after WM3 and before WM4. In fact, I don't even know if DiBiasi was in the WWF during WM3. If he was, it was at the very beginning of his time and wasn't big enough to be on the card.
And as far as WM4, giving the belt to Savage was the right thing to do because it accelerated the Hogan/Savage angle that involved Elizabeth.
There were two big reasons Savage got the belt at WM 4 - One, he had been with the company, a major draw, and done his fair share of jobs so you knew he'd be a company guy and follow the story no matter what. 2nd was the fact that Vince didn't want to mess up the marketing aspect of having a mega face champion pursued by heels, same formula with Hogan, plus the NWA had a major heel (Flair) as champ so it made sense to set yourself apart from the competition.
Remember, when Hogan took his next lengthy sabataical in 1990 the chose Ultimate Warrior to bo champ, not because of his great skill or teamwork mentality but because McMahon saw the Face Champ vs Heel Challenger dynamic as working. Certainly a heel Savage, DiBiase, or Rick Rude would have been good choices to hold the belt till Hogan's next return but Vince didn't want to change the formula.
Now Savage proved to be a better champion and draw than Warrior, his character lost a lot of its edge after being named champion and having to tone down a bit as the top guy of a very corporate kid friendly company. Warrior was also a losuy wrestler and the novelty of his squash matches wore off after a while. Say what you will about Hogan but he coud work pretty hard and put a solid match, albeit for only 15-20 minutes, when he faced good workers like Savage, Flair, Piper, etc. Warrior really couldn't.
DiBiase could have had a nice run as champ but as long as the NWA was selling with Flair McMahon wasn't going long term with a heel champ of his own. Certainly DiBiase couldn't have stolen the title at the big Prime Time NBC TV Special because Hogan wanted time off after WM, he couldnt regain the title at the big show.