If there was a simple answer to your question... I have used vapor barriers (VB|) inside sleeping bags for many years - only in very cold weather. Insensible persperation is what keeps our skin flexible and that is where the moisture you found came from. When the humidity gets high enough, the insensible persperation slows to nil or stops. Inside of insulation (sleeping bag), a properly used VB liner adds the equivalent of a few degrees to the bags rating - there is no question about that. Perhaps more importantly, it VASTLY reduces the amount of water that migrates into the bag's insulation (which condenses and freezes). On extended trips in interior Alaska winters, that kept my down bag functioning for weeks without extensive down time for exposing the bag so the ice crystals can (slowly) sublime away - important when I was packing up and moving every day.

But they are unpleasant to use until you get used to them and they really don't slow the persperation down that much if they are not sealed up properly (I have drawstring closures on the ones I made - like a people stuff sack 0 head and upper chest out, of course).

I have used an emergency mylar space blanket inside kids bags for them and it worked OK, but I'm not sure it was that much of a real improvement. I think you have to try this out for yourself and see what works for you and your body's particular metabolism and so forth. It may be good enough for you or you may want to find some other solution.

Now, if you're soaked and you wrap one around you to cut the wind and reduce evaporative cooling, they work pretty good. They have their uses. I'm comfortable enough with my experiences in general to believe that it is worth the little bit of space and weight to carry a couple of these, but they are just part of what I routinely carry for ready gear.

HTH

Tom