Vidic is a huge Man Utd fan.
I remember one interview with him, after his first international cap (he was playing for Red Star back then, some 4 years before he actually joined Manchester), and the interviewer asked him where in the big leagues he'd like to play. Vidic tried to avoid the question and at first gave a diplomatic answer, but the reporter insisted to get a more concrete answer and Vidic said that Manchester United is the club he'd like to play for.
Ever since he kept telling from time to time that ManU are his favorite team.
So if he's playing for his favorite club, if he's winning there, if he's loved by the fans, if he's getting paid a lot of money, if he's in good terms with Ferguson etc....why would he leave Manchester and why would Manchester want to sell him at all?
He's not gonna bring them so much money as Ronaldo did, not even close, so I doubt that it's gonna be some profitable transfer for Ferguson. ManU will spend big part of the money on another CB, and that new CB will be a risk, while with Vidic, who's "only" 28, you know that you get top quality and you know that he can play for many more years.
And why Inter of all teams? No other team in the world has so many quality CBs as Inter. Even if he's better than all the CBs Inter have I doubt that he's gonna improve the CB position too much.