Applying a vacuum to an area causes blood to well in that area, suck on your hand and you'll see the area you suck on blush. So presumably you're making capillaries open up and increase heat transference over the skin threshold. Opening all the capillaries in an area will increase the volume of blood in that area too, so greater volume and greater heat gradient (over a small distance).

Disclaimer: Assumptions, not medically/biologically trained.