Sony have announced that Playstation Plus will be available on Vita come this November and if you already have a PS+ account on your Playstation 3 it will be shared with your Vita at no extra cost.
Some features of the PS+ on Vita include:
Instant Game Collection: Unlimited access to a regularly refreshed selection of the full version of games as long as your membership is active.
Discounts: Take advantage of exclusive discounts on games and DLC for your Vita.
Online Game Storage: An additional 1GB of online storage for game saves for PS Vita games and downloaded content.
Automatic Updates: PS Vota will automatically start up at a designated time to download and install select game demos and game updates as well as system software updates and start standby mode upon completion.
Automatic Trophy Sync: Trophy information will be synched to PSN servers automatically via the PS Vita.