There is no real cross-play for NFS:MW. The closest is that your Speed Points will be cross-save, and Autolog, the asynchronous system of seeing your friends high scores, works for both, so you can see PS3-playing friends high scores on Vita and vice versa.
Ok so its not like all atars were you can use it like a controller darn. i think they should do that since the game its self is not two player but you can link with ps3 via vita and have a second player or more that be awsome lol. Probly be to hard and a lot of lag though.