That is incorrect. Most of them actually aren't compatible if I remember correctly.
Chances are, any PSP game you see on the PSN is 99.9% most likely to be compatible with the Vita. I have yet to run into a PSP game that isn't compatible with my Vita. As a matter of fact, most PSP games actually play over 9000x better than they do on the PSP. If you don't see a game in the Vita PSN store, but see it in the PS3 store, just download it Via the PS3 and transfer it to your Vita via usb. If you don't own a PS3, use sony's media go software and transfer from you computer.
For those that want to get the game directly from the PS Vita, most of the games aren't compatible, and new games aren't an exception, as Hakuoki still isn't downloadable on the Vita's PSN Store.
But that list still has more compatible than incompatible...
Not everything is compatible, period. It is a good concern to think if it'll be Vita-compatible before the release, it's not silly at all.
I have wonderful and surprising news, Corpse Party is back to #1! Because of the sale, it has passed Dissidia Duodecim, Monster Hunter Freedom Unite, Grand Theft Auto and it's the #1 PSP game from October! I'll take a picture soon to post here.
I saw that RO was also #2 (or was it #3?) for Vita downloads! Yay XSEED!!