2003 Pontiac Vibe check engine light for fuel vapor

I started having the Check Engine light come on a couple of months ago. The first time, we took it to Auto Zone, had the computer read and it said something about a fuel vapor leak or some thing like that. The guy said the fuel cap was probably not on tight. We cleared the message and it went about 2 weeks before the light came back on. We took it to AutoZone again, and it was the same warning. We decided to replace the gas cap. It went about the same time before coming on again. I don't want to just ignore a check engine light. Any suggestions?