The partial pressure of the fuel vapor in your tank is directly related to the temperature. At colder temperatures amount of vapors will be less and will occupy the same volume in the tank. The vapor volume is roughly 22 times the volume of an equal amount of liquid.
If the temperature is kept constant and a closed system is provided, there will be fuel condensation taking place along with equivalent amount evaporation. There will be a net result of zero change in liquid and vapor levels.