Chances are your Vibe is one of the many that have / had defective center caps that are / were prone to falling out.
There was actually a TSB (Technical Service Bulletin) issued for this very problem. The part number for the revised center cap is:
#20835338. According to
GMPartsCenter.net, the 2009-2010 Vibe and 2007-2008 G5 share this same center cap. I entered the part number on
eBay and there are a few that come up. As you can see, the prices on eBay are actually pretty comparable with GMPartsCenter.net and GM MSRP (dealer price).
I would highly recommend popping the other three center caps out and applying some clear RTV silicone to the edges of them before they fall out too. The center caps on both our 2009 Vibe GTs were siliconed before we took posession of them (new) and they have not budged. I have removed them a couple times for various reasons (re-balancing of tires, etc.) and I simply apply some new silicone and pop them back into place. When dry, the silicone turns rubbery so it won't harm the wheels or anything and is easily removed if need be.