When fuel is heated it vaporizes and expands as a gas, it's inevadble , even if the tank is empty or full the same process happens, only thing is when the tanks full the built up high pressure inside the tank tryes to escape once you crack the seal on the fuel cap (as the atmosphere has a lower air pressure then inside the tank)- my landcruiser does it, and u can hear the vapour trying to squeeze past the cap seal... I just do the cap up untill it bites then back it off 1/8th of a turn and their is never any pressure build up. If you pull the disc out of your tank that mounts the pump/ lines in & out you can see the vent sits flush on the inside of the disc plate.

Pop the line off the charcoal canister as samQ said, eliminate that, blow compressed air down the vent line. see if that works, if not just crack the cap ever so slightly.