Vibes came with a rear cargo cover, although I can't remember if it was standard or optional. It was a shaped vinyl thing that would hang from the tie down loops, the ones that are about the same height as the top of the rear seats.
If you're lucky, maybe the previous owner folded it up and stored it in the long narrow compartment just behind the rear seat backs.
Here's a pic I stole from an overpriced ebay listing of the OEM cover
I see aftermarket ones on the web, not sure how they are. For what its worth, a cover from an 03-08 Vibe or a 03-08 Matrix will work in your 06 Vibe