According to a listing on online retailer Zavvi Assassin's Creed III will be coming to PS Vita at the same time as the other versions of the game (October 31). It has been known for a while that Vita would be getting a game in the franchise, but not that it would be the next full instalment in the series.
No details are available at the moment, but it is likely that the PS Vita version will feature at this year's E3.