If anyone is not familiar with Playstation Plus, its a subscription that gives you access to a selection of free games to play across the Playstation 3, PSVita, and PS4 consoles, along with exclusive and additional discounts(specifically, a game that's already discounted to $10 is often discounted further to $5 for Plus members), even some cloud save storage. As long as your membership is active you keep the games you've downloaded, but once it's expired you lose access to them. I don't know if reactivating the membership reinstates access to those games after the expiration. Its optional on the PS3 but required for online play on the PS4.
If any fellow Vita owners are trying out the game for the first time, pass a friend invitation to "Imatron" on PSN, I'd love to lend a hand if I'm not busy.