I'm assuming that the vapor is either atomized diesel (which makes no sense to me) or crank case smoke (also not right to me) and that small line at the bottom of the tank you are referring to is the tank vent. you have the diesel pickup, the leakback, the vent and the fill. pickup and leakback are a closed circuit. Neither are exposed any kind of crankcase pressure that would pressurize your tank. if you're engine is idling and you remove the fuel cap, do you get vapor coming straight out of the fill?
Just to clarify, when you say gas cap, do you actually mean gas/fuel cap in the back, or oil fill cap?