that vacuum is created from negative pressure. when you pop the top on a vial, it has so much volume of both liquid and air inside. when molecules fill space, they exert pressure on the surrounding environment. when those molecules are removed from said environment, and an equal amount of matter does not replace what's been removed, the pressure differential within that environment changes.

thermodynamics tells us that high pressure will always migrate to low pressure areas in an attempt to equalize. hence the "vacuum" caused by removing oils without replacing the volume.