I've been browsing the playable games on the PS Vita listed here on the official website: http://us.playstation.com/psvita/games-and-media/#?platform=PSVita&Genre=Genre&SubCat=Vita Features&page=1
Every single game in this list is playable on the PSVita. You can click on any of the games and see something like "Platform: PSP®, PSVita". In most cases, there is a "Buy Download" option, which makes sense, as the PSVita cannot read PSP UMDs.
However, these are other cases where there is only a "Buy Disc" option available, and there are other cases where there is NO buy option available. For example:
1) Buy Disc only: http://us.playstation.com/games/kingdom-hearts-birth-by-sleep-psp.html
2) No Buy option: http://us.playstation.com/games/knights-in-the-nightmare-psp.html
Both of those PSP games list PSVita as platforms. "Buy Disc" is meaningless as it is UMD format for the PSP. So if you can't download either of them, and both of their release dates say "Out Now," what in the world does that even mean?
The above two are simply examples — the entire list is riddled with these mystery cases. How can I interpret this list? I can't buy a PSVita until I can concretely determine with no mystery what I actually can and can't play.
Best Answer
I've personally found that site to be quite poor. Selecting the buy now option for any of the digital games sends you to a different storefront altogether so they should probably work to retire us.playstation.com or tie it more closely into their actual storefront.
The real storefront is store.sonyentertainmentnetwork.com. The listings there are by far more accurate and your digital purchases should be applied to your PSN account. It is essentially a web-accessible version of the playstation store for all intents and purposes and should therefore be viewed as more authoritative. You'll still find some items you can't purchase, but most of those aren't released yet or actually are disk only (and clearly indicate as much). I'll admit that the presence of unreleased items in the system without purchase options and with inaccurate dates is bizarre, but it's better than us.playstation.com in terms of accurate availability for the things that are available. At the very least, the system compatibility has proven to be correct in my experience so if it says Vita, it should run on Vita.
Kingdom Hearts: Birth By Sleep is actually PSP Disk Only and not compatible with the Vita, and store.sonyentertainmentnetwork.com correctly lists that. Knights in the Nightmare on the other hand is available for purchase.
You can see a much more accurate listing of mostly all vita games there. I can't guarantee it has everything and the strange unreleased listings are certainly bothersome, but it is Sony's official storefront.
Fun fact: While they don't pull up in search results, there are ways that you can also find items that have been removed from the PSN.