As long as you 'add' the games to your account (add them all to cart & buy for $0.00, or hit Download on each them so it's in your download history) while they active under the PS+ promotion can you freely redownload and play them as long as you have a PS+ subscription.
The 'instant game collection' shit they introduced recently works in the sense they sub in/out a couple games each month, rather than an entire new list of games to try out.
Spoilered is a pic of this month's game collection state, which'll be changing presumably Tuesday, or the following one -
I tend to look at the system as extended rentals, where I can't really choose what I'm renting, but I tend to get my yearly subscription value out of it none the less. Does get annoying though when they put stuff up you paid money for prior.
As for the Vita side of things, no clue if they'll do the same thing as there isn't really many Vita games to choose from. I'd mostly just expect them to cycle in PSX/miniS games again, which they haven't done regularly in quite some time after moving up to proper PSN/PS3 games.