The codes for loose gas cap and O2 sensor problems are different and there is no interaction between the two. O2 sensors do get tired and 95000 miles is definitely in the range where failure is plausible.
Most cars have two O2 sensors. It's likely that the one replaced in 2008 is not the one that is giving you trouble now. The O2 sensors are used to fine tune the fuel air mixture. If one or both fail, the car will simply use a default fuel mixture and will probably run fine albeit with slightly greater emissions and perhaps slightly lower mileage.
OTOH, a few vehicles simply fire off O2 sensor errors at random. Our 1995 Dodge Neon reported one every six months or so. I simply cleared them and only replaced the sensor when the error persisted in coming back. But the Neon allowed me to read out a simplified version of the codes without a scan tool and to clear the codes by pulling a fuse. I don't think your Vibe has the capability to read codes without a scan tool, and I have no idea which fuse -- if any -- will clear the stored errors.
If you aren't going to replace the O2 sensor immediately, I'd suggest buying a cheap scan tool (about $60-$80) at Sears or Autozone that will allow you to find out what is turning the check engine light on, and to clear the codes. But there is actually a problem and you haven't fixed it, the codes will likely come right back.