Thermo ski boot liners are made of various sorts of closed cell foam with a light fabric liner. They are thus, to a certain extent, vapor barriers. The liner gets wet, but the foam cannot.
Not all liner fabrics in thermo liners are created equal. My Scarpa liners have a much thicker, softer liner fabric, which soaks up a fair bit of sweat. My Intuition liners have a very thin, insubstantial liner fabric, which soaks up very little sweat. I prefer the later for overnights, as they can be left out overnight and don't really freeze up much.