Cross play for ps3 and psvita

From the PS Vita Cross-Platform Play Playstation Knowledge Center article:

Cross-Platform Play (sometimes referred to as Cross-Play or Cross Play) is a feature that lets PS Vita games interact with compatible PlayStation 3 games. The type of interactivity varies by game, and not all games feature Cross-Platform Play support...

Cross-Platform Play is broad term that covers any PS Vita game that has the ability to interact with the PlayStation 3 version of the same game. New and exciting ways to utilize this unique technology are continuously being introduced;

Each game implements cross-platform play differently. You'll have to investigate how each game you're interested in takes advantage of and implements its Cross-Platform Play feature. The FAQ article above mentions several examples:

• Marvel Ultimate Marvel Vs. Capcom 3 allows you to use your Vita to control the game running on a PS3.
• Wipeout 2048 allows you race online against PS3 players.
• MLB 12 The Show allows you to transfer game saves between the Vita and PS3, so you can essentially play the same game between the two.

Games that support cross-play will have this icon on its box:

If you're talking about being able to multi-player games with PS3 and PS4 versions of the games with your Vita... here's Sony's Cross-Play on PS Vita FAQ's. You must own all of the various versions separately in order to do this. So for instance with WipEout, you must own WipEout 2048 on the Vita to be able to play against someone playing WipEout HD on the PS3. It doesn't matter if the game is a physical or digital copy.

What you're describing is Cross-Buy, not Cross-Play. Cross-Buy is where purchasing a game on one system (be it PS3, PS4, or PS Vita) lets you play it on another. Cross-Play, as Splattered Bits points out, is the ability for a game on one system to play online with the same game on another (i.e. Vita and PS3, as you brought up).