Pressure/Vacuum printed right on the cap is a dead give away that this is NOT a vented cap. If the truck has the "Evaporative Emmission System", then the tank is vented thru the charcoal canister under the hood, and a non-vented cap is correct. The vent line is plugged, if this is the case.
If the truck was not equiped with the "EES", there would not be a CC under the hood, nor a vent line running from the tank to it, so a vented cap would be needed.