What you have to realise is that the Vita hasn't had massive development like the 3DS has had. Because of the unsuccessfullness of the Vita, only a tiny amount of devs are actually doing stuff for it, and not a massive group of hackers/devs. So it's really limited the general public as to what you can do with the Vita, hack-wise.
To answer your first question, No. There is no way to downgrade the Vita at this moment in time. You basically have to find a good, low firmware between 3.00 and 3.51 (the golden firmwares for hacking at the moment). Much lower firmwares (we're talking about <2.81 here) only have a tiny amount of exploits for them (web browser and PSP/PSX-side exploits, I think) and aren't very suitable to start hacking on. Whereas firmwares between 3.00 and 3.51 have the fail-mail exploit (not for 2.XX and 1.XX FWs because of drastic changes needed to execute the exploit) and much better custom bubble and PSP-side kexploit methods. TL;DR, get a 3.18 or a 3.30 Vita at the most. They are the absolute golden firmwares! You can take a look at what you can do on those firmwares
here.
To answer your second question, No. The Vita hasn't been cracked enough to be able to decrypt even a save file! However, you can use PSP savedata exploits to install PSP bubbles onto the Vita, bubbles such as TN-V and ARK (both PSP game/homebrew launchers). These savedata exploit methods are kind of redundant on <3.52 firmwares now though, since you can use the fail-mail exploit to patch any PSP game/demo so it launches TN-V/ARK instead.
The Rejuvenate hack is the closest you are ever gonna get to native Vita homebrew. You need to be on 3.51 and under to use it, and it involves either PSM Dev (now unavailable), or PSM Dev for Unity (Still installable through
this method.). Then you will need to set everything up through
here.
Ah, I also forgot to add that the Vita, at the moment, does play pirated PSP/PS1 backups, but not Vita backups.
