Just thought I would join in on this iPad vs Vita debate!
I have an iPhone 4S, iPad 2 3G and also a Samsung Galaxy SII as well as a Wifi Vita. Now I wouldn't consider myself a fanboy to any particular product, as I can see the strenghts and weaknesses in them all, and can enjoy one without having to slate the other.
In my own view, the Vita is by far and away the best way to play games. The screen is magnificent, I think on a par with the Apple products. It may not have a retina display a la iPhone 4S, but, in all reality, this is not needed. The screen of the Vita is sharp and clear, the colours stand out magnificently. Much like the Apple products.
So screen aside, yes, the iPad does a hige number of things that the Vita does not do, and some things better than the Vita - eg browsing the internet. I love my iPad for reading books, browsing the web, emails, etc, etc, so for these things it is much better than the Vita.
But I got the Vita for gaming. I like to play small games, like Angry Birds, from time to time, and the Apples are great for this. But I LOVE playing proper games, and not having the time to play on the PS3 as much as I would like (what with having a wife who watches the TV or else my son is on it), the Vita suits a treat!
I have tried playing games on the iPad, including the Final Fantasy ones and even fighting ones, and they do look good. I don't think they look as good as the Vita, but they do look good nonetheless. But, and a big but it is, they don't play great. Touchscreen controls on an action, RPG, driving, or indeed anythiing other than point and click or simple games is not a joy. A royal pain at times, which takes the enjoyment away from the games, thereby destroying the sole reason for playing games - fun. I am good with touchscreen devices, can type at speed on them, so I don't believe it is me not getting to grips with these devices, it's just that it's not fun playing great games on a touchscreen only.
So for gaming, the Vita wins hands down. I think other posters have got it right - these devices are not really comparible. I don't think comparisons to internal specs are needed, nor differences in prices of the gadgets or games. If you want something that you can do a number of things on, go with the iPad. If you want a dedicated gaming device that can do a lot more besides, it's the Vita.