Vai may be able to do anything Mayer does TECHNICALLY but what Vai can't do is play tastefully. Mayer has that down to an art. And in that respect, Vai can't hold a candle to Mayer.

Example:

By this video, Vai has no grasp on tone either - that's why he gives it away!

http://www.youtube.com/watch?v=B2JycfZ23qs