When you buy a PS3 game such as PlayStation All-Stars, you will be able to get the PSVita version for absolutely nothing. Yes, free, meaning you pay once but get to play it on both systems.
Other confirmed supported games include Sly Cooper: Thieves in Time and Ratchet & Clank Q-Force. Though we assume they will include more games in this deal.
Sony indicated that the free game will only be available when buying the PS3 game and not through purchasing the PS Vita version.