That's a common problem. For me I experience it most often if I leave the Vita on sleep mode and return to it after a prolonged period of time. Usually when this happens, I'll do a force reset. However, if the problem is persistent (this happened to me A LOT when I was playing P4 Dance All Night), I did a format + restore, also made sure my firmware is truly updated. Ever since this, the blue screen of death hasn't been torturing me as frequent as before.

Btw, how new/old is your Vita?

If you're able to get a save point I think it would do the Vita good to power it off completely. Probably the equipment is not as optimized as we'd like to think it is. Haha! Yes I do experience slightly weird graphic defects too prior to blue screen, sometimes the screen would tear, especially in World of Final Fantasy. Heck, it's annoying but doesn't provide me a good enough reason to get a new one. I wonder when will Sony announce a new handheld!