This isn't a bad idea. You could get a PS2 slim, which will play PS2 and PS1 games. You already have a X360, which is essentially a better version of the PS3, and a PS4, which has several remastered PS3 games.
That said, Demon's Souls alone is worth ponying up for a PS3
Are the servers still up for that game?
OT PS3 has more to offer overall, but VITA has Persona 4 Golden. Do what you will with that info.