The only thing that's for sure is that yes, a ton of people in the company believe it (the rumor was initially circulated online by people in the company), based largely on the idea that he must have done something really, REALLY bad for Vince to be so negative towards him. For the obvious reasons (Vince's classy announcement on Raw and it was well-known for awhile that Savage was unhappy with his spot, which has recently been backed up by Bret Hart's book), if it's true, I don't think that Vince knew at the time/there's any chance that Savage was fired for it. Personally, I tend agree with the reason for thinking that it's true but I have no idea if it's really what happened. At this point, the semi-official "he's too crazy to deal with" story doesn't ring true (it's wrestling, it's Vince, etc.) for even though he's surely pretty crazy.
Also, for what it's worth, I just took a look at Bret's book and as far as the jump, he said:
- Lanza told him that Savage drunkenly called Vince at 4 AM to let him know he signed w/ WCW.
- Vince and Lanza were very upset about the jump and were in tears after Bret said that he he'd was loyal to Vince after noting how he was the only real boss he ever had besides his dad.