Reviving this thread to talk about putting 2005-2006 16" Matrix alloy wheels on a first gen Vibe.
I got Matrix 16" alloys for $200 from Kijiji buy and sell. Had to buy mag-style lug nuts, so went the eBay route and bought a neo chrome finished set, they look nice. Also got Pontiac center caps from eBay. It's true the Pontiac center caps don't secure in the center bore holes, but that was overcome using rubber strips to pad them out (details below). It has been said many times Matrix wheels will mount perfectly on a Vibe but for me that will be proven once the ice thaws.
I experimented with padding out the Pontiac center caps with rubber strips so they will secure in the Matrix alloy wheel center bore. Basically I put a broccoli elastic around a plastic cap from a Listerine bottle and jammed it into the center of the Pontiac cap to give the cap's plastic tabs rigidity (perfect fit). I sliced the excess off the plastic bottle cap with a kitchen knife . Then I wrapped the outside of the Pontiac cap's plastic tabs with a short cut piece of v-groove rubber belt I had on hand (Hutchinson #1344051000). Then another broccoli elastic around the v-groove belt to hold it in place. This rubber and plastic sandwich flexes to fit the alloy wheel bore hole yet has enough structure to fit securely. The preceding is not a how-to, just a general idea to pad out the Pontiac cap with rubber to secure it in the center bore hole. I will report back after 1000km.