Do the Pontiac Vibes have the same dash as the matrix? I know they were essentially the same car, but not sure about the dash.
Does the radio in her matrix tout that it's Satellite ready? Is there any indication that there's an auxillary setting on it? If so it may have an IP-Bus, and if that's the case it's as easy as picking up an IP-Bus to 9mm cable, removing the dash/head-unit, hoooking upt he cable, deciding where you want it to come out and baddaboom baddabing you're in business.
But anyway, why not just go aftermarket and get a dash kit and a wiring harness and call it a day?