I've never used a vapor barrier liner, but have always been intrigued by the idea, worth a test run at least. Some people swear by them and some hate them. My thoughts are that a vbl will keep you bag's insulation dry as your sweat won't collect inside the bag. With a gore tex bivy sack the bag should stay dry from the outside and the combintation should keep one warm and dry and likely extend the warmth of the bag by itself. Good in theory anyway.

I read that during Will Stegar's north pole trip the bags they used gained so much internal moisture buildup the bags wound up weighing 40+ pounds each. The duration of a trip surely plays a large role here since each night an incremental amount of sweat will accumulate in the bag and freeze. On a weekend trip it may not matter.

A few web links on vbl's:
http://www.verber.com/mark/outdoors/gear/clothing.html#VaporBarrier A good write up and some additional links.

http://www.warmlite.com/vb.htm Very opinionated, take it all with a grain of salt.

High end commerical vbl's:
http://www.westernmountaineering.com/ind...mp;ContentId=44

http://www.integraldesigns.com/product_d...producttypeid=5