As announced at Gamescom, PlayStation Plus is coming to Vita. It will be available this November--and best of all, it will be available at no extra cost to current subscribers. That means one fee will cover both PS3 and Vita.
Like the PS3 version of the service, PlayStation Plus members will gain access to an "instant game collection" for Vita. A regularly rotating selection of full games will be accessible, so long as your membership is active. In addition, Vita owners will get an additional 1GB of cloud game storage. Finally, Plus members will be able to schedule automatic updates and trophy syncs--just like on PS3.
A PlayStation Plus subscription is currently $18 for 3 months and $50 for one year.