Since the VFD "receives" serial data from the Empeg and the Onstar "sends" data to the empeg you can have both connected at once - all you need is a TTL signal splitter cable - with one wire (plus ground) going from the onstar to accept incoming serial TTL from your Onstar and send it to the Empeg serial port via the MAX2*, and the TTL serial out is sent to the VFD unit.

You could even place a jumper switch on the board to allow you to "route" the TTL level GPS data to the VFD allowing you to display the GPS data (assuming its in NMEA format) on your VFD as a form of system "self test".
[I think the VFD will accept regular ASCII data and show it as it has onboard Character Generator logic].