Select 'Open decryped' from the menu and press X
I can't say, if there are any aftereffects, so please - for the love of god - backup your savegames, if you hold them dear!
The whole process is quite easy, but I must warn you, that I haven't tested this with much detail! The game loads, I can select my copied over savegame with the whole content I unlocked and I can start a game with that.
I have been playing BoI:R on Steam for quite some time, but I recently bought a Vita and was delighted to see that BoI:R is available here, too! I have my Vita just a week now, so bear with me, if I say something stupid This is a small guide on how you can copy over a savegame from the Steam version of 'Binding of Isaac: Rebirth' to your Vita.