Rockstar I'm sure by now has realized that portable games aren't their forte. GTA Advance was "meh". LCS and VCS were good for their time but are incredibly bland and shallow by today's standards. Chinatown Wars was great, but for a portable game.
While the PS VITA's specs could well handle a 3D GTA game, it's sales aren't high enough. The 3DS, however, can.
What would be interesting is a remake GTA III for the 3DS but with full online capability and a robust amount of different modes.
Grand Theft Auto IIIDS