Yes. See list in the database in the VX_8R Yahoo group. This link may work: http://groups.yahoo.com/neo/groups/VX_8R/database/1/records
(yes, Yahoo has switched me to their new interface). I have personal experience succeeding with Plantronics Voyager and Voyager Pro Plus headsets. Several other headsets were partial or total failures. Yaesu's implemenation of BT was weird to start and has not been kept up with the current standards. I am not aware of any wired headsets from 3rd parties; you'd have to buy a connector/cable (several online vendors have the correct connector with cable & bare wires) and wire your own adapter.
I've used the GPS antenna on the radio & on the speaker mike. I prefer on the mike - for my needs, seems sturdier and the mike is more likely to be where there's a clear view of sky. But both locations work well enough and on radio is fine if you are not using the speaker mike at all or use a wireless headset.
I do not think the VX8 TNC is capable of being used to send/receive other data with an external terminal. You could wire up a connector and use it like any other radio with an external TNC or sound card software. I think a dedicated data radio (used HT, one of the cheap Chinese throw-aways, etc.) would be better. There are many examples on the web of folks packaging a radio, tnc, GPS, and power for a portable digi, data terminal, or tracker. Check with your local emcomm groups to see what, if any, data modes they are using and what you'd need to work with them. Re: PSK - an FM radio (HT) would produce/receive an FM signal modulated with the PSK tones.
My experience with a several years old (upgraded by Yaesu to current firmware) VX8 on APRS is the deviation of the signal is low compared, at least, to what is normal in the metro Washington DC area (including properly configured wide area digis run by hams with a clue how to do it correctly). The VX8 is on spec, but success with just 5W on a busy APRS channel depends on being very close to a good digi, internet gateway, or whatever station you want your position displayed. A good antenna will help but may be insufficient due to congestion, terrain, or distance. In use as a bike mobile supporting century ride events, I've had better results with a TinyTrak3+8W module (Byonics sells it as a single board), and excellent results with the VX8 and an outboard amp at about 20w output to a ground-independent antenna (Larsen 1/2 wave whip or Comet 5/8 wave whip).
You can probably get more informed answers in an Emcomm/ARES forum and the VX8 Yahoo group where there's more experience with data modes in Emcomm and with the VX8. 73's de KS1G