There are few people on the planet with more cold weather trekking/camping/expedition experience than Lonnie Dupre
. Lonnie was a presenter at the Winter Camping Symposium in Minnesota. When I asked him how he dealt with the buildup of frozen moisture in sleeping bags for day after day of travel in severe cold, his answer was (paraphrasing)...use a vapor barrier between sleeper and insulation. In his case, he slept in a single layer of baselayer garment using a vapor barrier "inner bag" separate him from the several inches of down in his sleeping bag. The morning routine involved a quick change into a dry baselayer before layering up for the day.
Vapor barriers can be tricky to manage, but for serious cold they can be a lifesaver.