What service would they be providing VATSIM pilots? Same as in the world (opening and closing VFR flight plans, providing weather information, etc.) or did you have other or further tasks in mind?
FSS Staffers Would have the normal duties of an FSS control, plus that of Flight Watch. They will have access to a radar screen in order to assist in determining the exact location of an aircraft. However they would not provide vectors out of severe weather.
FSS may choose to serve an entire ARTCC, or just one FSS. For example, in ZKC there are the McAlester FSS, Jonesboro FSS, Columbia FSS, Wichita FSS, and StLouis FSS. During busy times in ZKC, an FSS Staffer may only service Columbia, Wichita and StLouis FSS. During less busy times, an FSS staffer may service the entire ARTCC. Staffers may share service, even if they are operating the same FSS.
Comms are not easily set up for this type of situation. Some VORS have two-way comms, on a different frequency, some receive on the standard 122.1 and transmit on another, some receive and transmit completely differently. I know that vPilot supports transmitting on one frequency, and monitoring another. I also know you can listen in VRC to multiple freq. and alternate your transmissions. Staffers could even log in, and not appear as a new controller, instead listening on all or just a few VORS, waiting to respond. Or, you could opt for a more simple system, one where all pilots transmit on 122.1, but listen in on their closest accepted VOR, which would be listed in the controllers information. This would assist the staffer in determining the aircrafts location, while assisting in keeping things simple.
I am testing some of my theories on VATSIM right now, Ill get back to you.