I don't like Vucevic much. Just don't think you can win the games that matter with a defensive liability at center. Some teams can be exceptions if they have transcendent talent elsewhere on the floor (Miami for example) but in general you need that protector in the paint.
There is a definite difference between the words "liability" and "average".
I know what the words mean, and I picked 'liability' for a reason.
The Magic were a better defensive team last year when he wasn't on the court (they sucked either way though). He posted a very nice PER of 18.6... while opposing centers posted a PER of 19.8 while he was on the court. In other words, he takes away more than he adds.
I think Vucevic is absolutely an elite rebounder. I think he's a good opportunistic scorer as well, though I'm not sure he's the kind of player who can reliably generate his own offense. But he's not an asset as a defensive player, and center is the most important position to have a defensive asset.