Most if not all of the flip type caps ARE vented. I run a 3/8" fuel rated hose from a fitting (with a check valve) from the top of the tank into the filler neck about 4-6" below the cap. The air is then forced from the tank as it fills and still has a vent when the cap is closed. If the tank was not vented in some manner it will collapse as fuel is pulled out by the pump. The check valve in th line and another in the tank are there to prevent fuel leaving the tank if the tank is inverted (I think I would have other things to be concerned about too in that situation
) No vapors in the garage at all.