Every now and then one of these guys is bound to say something that just makes Vanitas either laugh at their stupidity or it just baffles him. Right now it's definitely the later. There's really no way for him to be something other than darkness and shadows. That's what makes him. Yet here Sora is, yet again, trying to help someone be more than just what they are.

Really it's no wonder he and Ventus had bonded so well. Either way the expression Vanitas is wearing is one of surprise. He has to look away to laugh, but there's no bitterness or mirth in it. It's forced.

"Sure, little brother. You go find a way and then maybe I'll actually consider it."

Only because there's no way he can succeed.