Demon's Souls is well worth it.
As you already know the PS2 classics are also only on the PS3, though Vita can play PSP classics (but PS2 > PSP)
PS3 HD also going to hold every single classic you want, Vita will demand those proprietary cards.
The only reason to pick the Vita over the PS3 would be if all you care is portability.
The Vita has a better version of Persona 4 with Golden but a lesser version of Persona 3 with Portable.
Even there it would lose, IMO, as Persona 3 is the best of them all (including the masterpiece you are playing right now) and you would want the full experience plus a free DLC of it rather than Persona 4, again, IMO.
So definitely go with the PS3 if you don't NEED to play on the go.
You could get both, tho, if you want it all.