The JVS board lives in the control panel on a vewlix, and is for connecting the control panel wiring harness (buttons, stick) to a JVS game, such as the Sega Naomi.
The Fast IO board sits in the same spot (in place of the JVS IO) in the control panel, and is for connecting the control panel to FAST IO game (basically something Taito Type X2 or 3). The JAMMA connector, audio, and video connections remain unused when installed in a Vewlix, although theoretically you could use them to adapt a JAMMA cabinet to run a FAST IO TTX2/3. (I might be wrong but I think they were originally designed for the Taito Egrets to adapt them to FAST IO)