That doesn't compute - gas is being pulled out of the tank reducing its volume relative to the volume of the tank - normally air has to flow into the tank to take the place of that displaced gas.
THAT is the purpose of the vent line. (but also if the tank gets hot - as in summer or from engine heat, without any gas being consumed, gas vapour can be released and this is also dealt by with the vent line and why CA systems have the Evaporator Canister)
If the vent line is plugged, then no air can flow into the tank to replace the displaced gas and it creates partial vacuum.
If you really do have the tank pressurizing positively, then somehow you are leaking compressed air into your fuel system, possibly from your forced air system?
You can do a test if you drill the hole in the cap fixture to open it to the vent - then put your finger over the hose and see whether it is pulling or pushing.
If you are creating positive pressure in the tank, then it might suggest you have bigger problem than it just not venting