Sony Worldwide Studios boss Shuhei Yoshida has confirmed that Vita can only be used as a PS4 controller if specific games have that functionality programmed into them.
A Twitter follower asked Yoshida if it was possible to use a Vita as a second controller for multiplayer games.
He replied that it wouldn't be possible unless developers added that functionality for their game.

http://www.computerandvideogames.com...ly-programmed/