Vapour barriers are the cold weather solution. It may be theory or practice, but it has been around a hell of a long time, from the Mickey Mouse boots they used on everest in the 50s (which was just army stuff from before), to VB technology today. Basically vapor is produced by the body, and moreso in cold dry weather. If the bag is warm it has to condense in the bag, if the bag is cold, there may be enough heat to drive it out. VBs do three things: protect the bag; reduce your water loss; and make you feel a whole lot warmer.

The only downside is a that they can make you sweat too much if you are doing something with spikes of effort. So sleep is ideal for them, high altitude climbing with a consistent slog rate is another possibility. Gear with lots of ventilation potential, like under arm zips is good.