I agree with Milan, I don't.see a difference with the.Vita's screen. I think SD.looks.just as good. Now, with my Xperia Ion, that's a different story since my screen for that is beautiful.
I am assuming the only reason it looks better is simply because your Xperia has a higher resolution than the Vita and a higher pixel density (ppi). Other than that it's a basic LCD screen with a higher resolution. If the Vita had the res and ppi on it's OLED that your Ion has, no question it would look far better obviously on the Vita but it doesn't have either just a bigger screen.
Not like you probably didn't know that I just felt like talking about technical specifications for no reason really.