The vent is in the fuel fill assembly itself. The cap has a small vent hole and a rubber check valve (plug) on its underside.
Here's how it works: The fumes or overflow run up the vent line into a fitting jsut below the top of the filler. From there they flow back into the tank or vent to the atmosphere in the case of fumes. If you fill the tank all the way until the filler rig shuts off, you will probably have some overflow or may block the vent. You should try not to completely fill the tank (easier said then done in many cases)