I'll buy one to steam ps4 games to, but I can't imagine how bad Vita games would look blown up to a 4K screen... which will definitely be a standard resolution a few years into the next-gen cycle.
PSP games had remasters (God of War) and PS2 games as well, those looked just fine. And also, 4K screens won't become a standard for a while. They're still expensive, and most people are happy with just their normal HDTV's right now.
But anyways, the Vita already upscales to 1080i. They definitely won't look that bad.