putting the vapor barrier on the outside of the bag in the winter is a BAD idea. All of the moisture coming off your body in the form of vapor will freeze inside the loft and if you need to use the bag for several days, your 3 pound 20 degree bag can become a 15 lbs block of ice with zero insulation.

It's the same with boots. I use Integral Designs vapor barrier booties over top of my socks and inside my big big winter boots to prevent the felt liners from getting wet and freezing.

On a side note I wonder if the a GI poncho liner would work well with my silnylon tarponcho.