Remember when Sony announced that they were going to make most of the PSP library Vita compatible? Well, they're taking their sweet time, and a lot of high profile games still aren't on the Vita storefront.
As of today though, you can finally pick up Metal Gear Solid: Peace Walker for use on your Vita. If you already bought the game on your PSP, just load up your Vita and download it from the PSN.
It's a bummer that Peace Walker HD never came out on the Vita, but this is the next best thing, and for many people, the only way to experience the game (which I highly recommend by the way). For those who are interested, note that Transfarring will not work for this version.
Metal Gear Solid [PlayStation Blog]