People are vastly underrating Bisping (as always) and overrating Vitor. This fight is a pick 'em... if it wasn't in Brazil, Bisping would be a favorite. He's a better wrestler and striker, he has much better cardio, and Vitor probably won't threaten him off his back.
Vitor is faster and could definitely swarm and finish him, but don't be surprised if he gets tooled for 25 minutes.
No one is underrating Bisping.
We all realize he can win this. But it isn't a favorable matchup like he is usually given.
In my opinion he is not even close to a better striker. Vitor's hands are much better than Bisping. Bisping does a good job with footwork, and breaking a guy down over time. But Vitor has much better boxing when fresh. He puts almost everyone out that isn't named Anderson.
It just isn't a great matchup if they are trying to get him a title shot. "hey come down to Brazil and fight a 1st round monster in his home country".
It isn't like Vitor is a huge favorite. And no one really has said he should be. People that say Vitor easy just don't like Bisping. If Vitor wins it may look easy though, as it will probably be in the first round.