Well, I don't think it'll be "the end" per se. vBulletin currently has a large following (which is diminishing rapidly), they can pick themselves off the ground if they want to and I'm sure that Internet Brands will make sure they do. I'm sure they dished out a lot of money, they aren't going to lay down and die.
They have plenty of licenses out there right now, they make too much money to let a competitor take over just like that. I'm not sure they know this, but if they had never made 4.x complete ****, people would still be using it far more.
I think xenforo is clearly the future of forums. They will become a new powerhouse alongside vBulletin and IP.Board.