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.