Just look for an online match in either game.
I don't think you can just simply connect two players in the same room (one on the Vita, one on the PS3). The Cross-play basically allows the same game on the PS3 and the Vita to play online with other players regardless of which device they are using.
I know with Wipeout 2048 / Wipeout HD/Fury, you can invite other players to your game through PSN. I'm pretty sure most other online games work this way, as well. If you are trying to play with a sibling or friend who is on the PS3 while you are on the Vita, just invite them to a game and see if you can connect that way.
I don't currently have my Vita (sent to Sony for service), so I can't check myself right now.