Stephensen says this:
"Humans have a problem which we are told other animals don’t have: the moisture IN our skin evaporates in dry air, thus losing heat and water. That moisture loss is called “insensible sweat”, which term, like “military intelligence” is an oxymoron (ie, self contradiction). That “insensible sweat” is NOT sweat, and IS sensible: you FEEL it cooling you (but don’t feel it as wetness, thus the “insensible”)."
Insensible sweat is just simple evaporation of water molecules from near the surface of your skin into the air. There are no sweat glands of the insensible type that turn off and turn on for insensible sweat. It is just physical evaporation.
The process is described as evaporation, not as perspiration being exuded as water through the sweat pores.
Sensible sweat is plain old water being exuded from sweat glands and it comes out of your sweat pores as liquid water.
Water sitting in a bowl is similarly evaporating directly into the air, and ordinarily, the bowl will become dry when the water has completely evaporated. There are no perspiration pores in water similar to sweat pores connected to sweat glands in the skin.
However, if the bowl of water is completely enclosed in a waterproof bag, when the vapor pressure of water in the air is equal to the saturation pressure, no more net water will evaporate off of the water surface. [actually water molecules are moving out of the water to the air at the same rate that water molecules enter the water from the air ... so the NET evaporation is zero even if the skin to air evaporation is greater than zero because that is canceled by the air to skin condensation. It is correct to state that the NET evaporation can be zero but the evaporation from the water to the air is not because it only needs to be balanced by air to water condensation for the NET evaporation to be zero.]
However, if the surface of the bag is cold, the water vapor will then condense on the cold bag. This will drop the water vapor pressure and the water will commence to vaporize again .... and you end up with a stream of water vapor running from the bowl of water to the cold bag surface.
[technically, you reach a kinetic equilibrium where the rate of water molecules passing from the skin to the air is the same as the rate of water molecules passing from the air into the skin. NET = 0. For the bowl of water, substitute water bowl for skin in the previous. Just physical evaporation and condensation. Kinetic theory & physical chemistry are one place to see this.]
If the bag surface is warmer than the bowl of water, this bag condensation cannot happen, and there is no water vapor streaming from the cooler bowl to the warmer bag and condensing there because it is too warm to condense there.
If the bag and the water bowl are pretty much the same temperature, whatever water vapor streaming there is is diminished. At a small enough temperature difference, we can say that the evaporation-condensation water vapor transport system effectively stops.
Replace water bowl with water containing flesh and repeat the argument about water vapor transport to the bag wall.
If there isn't a good deal of insulation on the other side of the Vapor Barrier, then the Vapor Barrier bag will be cold and will be an excellent location for condensation of water vapor, and you will be effectively pumping water vapor onto the Vapor Barrier liner where it condenses and makes you miserable and mad and a hater of the idea of Vapor Barrier stuff.
You could get into a situation with a very thin sleeping bag and a very cold night where the (cool) Vapor Barrier liner turns into a cold clammy sweat box. It could be dangerous. But, so is it dangerous with just the very thin sleeping bag on a very cold night.