The Vibe is mechanically identical to the Matrix, but I heard GM use their own electronics or something.
I think the Vibe looks slightly better and is a bit cheaper, so it's a good deal.
My parents bought a 2005 Matrix almost a year ago and, even though we were considering the Vibe, the price differences in the used market weren't that big, plus those stories I heard of the Vibe using GM electronics and not Toyota ones kinda worried me.