I think I'll hijack this thread, rather than make my own, since my situation is pretty close to OP's (before he updated his vita lol).
I had some interest in vita hacking about a year ago, so I'm on 3.15 and I have the patapon 2 hack installed and working (I believe TNV is working, I get the PSP main menu, and I remember some white-on-black text for a "total noob" menu or something). I also still have a ps3 that I can use if necessary. I never actually got around to loading any isos or homebrew for the psp on there though, I lost interest. And I recall at the time that for PSX iso loading, I needed a separate ps1 game, so that's not working either.
Now I'm coming back, and it seems like there's more options. The advice in this thread is not to update to 3.55 or 3.51 or any version past 3.18 (again, I'm on 3.15, not sure if the difference between 3.15 and 3.18 is significant enough to bother going through the complicated [and therefore risky] "select an update" process). I've found
http://wololo.net/rejuvenate-ps-vita/, and I've found
https://gbatemp.net/threads/3-55-tn...previous-hack-necessary-ps-tv-or-vita.399955/, but I haven't found a modern guide for 3.18 users.
If anyone has a guide that can get me running ps1 games, psp isos & homebrew (I guess I'd like to update TNV, my version is "3.18 TN-V10"), and anything else available, that'd be swell.