What accessories are used with this thesterious accessory port

No sorry, but Sony have said they are aiming to make as many compatible as possible. Heres a list of all the initially compatible games.

I found this FAQ which answers a few questions but I'm not sure if this is the definate answer you were looking for. It says (about half way down):

Q: Is PS Vita compatible with other PlayStation platform games like PS3, PS2, or PSP? Do you have any plan to support other PlayStation platforms in the future?

A: Users can play PSP titles, minis and PSone classics titles which are offered on PS Store. We do no have any plans to make PS Vita compatible with PS3 or PS2 titles at this point.

Q: Is PS Vita compatible with PSP content other than games such as video and comics available on PlayStation Store?

A: SCE plans to make those content available on PS Vita.

Q: Can PS Vita users continue using their save data for PSP games that they have also downloaded from PlayStation Store to PS Vita?

A: We are looking into the opportunity to do so.

The bolded would suggest no, only confirmed titles would be supported however this is really vague and still doesn't completely prove or disprove that the use unsupported games will work. However Sony have already said UMDs aren't supported and if the only way to get the PSP games onto your Vita is through the store then I highly doubt any unofficially supported games will work.

Sorry for not providing any concrete definiative sources I just couldn't find any.

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.