The vapour return line vents from the tank as a safety precaution and probably EPA regs as well. I think there is meant to be some sort of breather vent/catch can thing that is mounted by the tank from the vapour return line (it looks like a tiny white fuel filter). The tank vent is meant to go into the air cleaner box from the charcoal cannister. You shouldn't be losing any fuel through the vent line or anywhere else. If the carb you're running has a return barb on it I would recommend you keep it connected. Don't know what to suggest - maybe open the fuel cap and run compressed air through the vapour line to check for a blockage? Take the cannister out and see if it is full of fuel or blocked up in some way (?).