I am really a noob with psvita so here is my -obviously stupid - question.
Can I use vitamin to decrypt a vita isos (the ones you find on ThatISO site)???
Nope; If the ones on That ISO Site are in VPK format, they will already be ready to be installed and played! This is usually the case anyway; I haven't come across a single backup on any sites that needed to be decrypted...