Stann and Vitor are different animals. I will admit I also thought Stann could possibly beat Bisping but I was not very confident about it since Mike had the better speed, cardio, technique and even better grappling. But against Vitor IMO He doesn't have any of those advantages outside of cardio/pace. Vitor is faster, a better striker and his ground game is more developed even if he doesn't use it very often.
I just think Vitor is a bad match for Mike with is hand speed and power. I expect Vitor to blitz Mike at some point.
It's not unconceivable to think that the big punch doesnt come though, and Bisping could outgrapple him against the cage, land a jab every now and again, and take the decision.