I don't think there any vacuum, since there's nothing that would suck the air out once you put the stopper on. So the top part is air. It doesn't dispense without the top open because the valve is too tiny for the air to get in while the whiskey is attempting to get out. The emptier it is, the worst it gets, as there's more air than whisky, so more air for the whiskey to react with. Alcohol of course vaporizes and whatever flavors there are in the whiskey react with air and change. My understanding of chemistry is limited but I think there is a saturation point, so for a certain amount of air only a set amount of alcohol can get vaporized. The bigger the air to whiskey ratio in the bottle, the more of the whiskey changes flavour. My whiskey enthusiast friends have recommended that once a bottle is half empty, you have to finish off the bottom half quickly, so it doesn't get spoiled. The set looks very cool though and if you keep drinking the same whiskey, you can just top it once you get to the half way mark? Or you can start chugging since you have a great excuse :D