Back in my ground dweller years, when backpacking in serious cold I'd carry a vapor barrier...it doubled as my pack liner and was long enough for a complete me to get into it. It was a pain sometimes loading the single cavity frameless pack (a Mithril by Moonbowgear) because of the excessssssssss material this pack liner had. That said, it was sil-nyl and as a true vapor barrier was/is designed to go directly against the skin, e.g. a naked body. With this method another potential 20F could be had and NO moisture would get on clothes or inside the sleeping bag because 1. it was eliminated from formation by stopping the process of insensitive sweating and 2. because this vapor barrier being sil-nylon was impervious to moisture.
The only B%^&@ about using it was in the morning....picture yourself waking up in a very very warm VBL that is also moist and having to climb out of it naked at anywhere from -8F to 10F !!!!
Of course you would transition from this VBL as FAST as humanly possible.
This pack liner therefore performed several functions-pack liner, entire body VBL (I would cinch it around the neck and have balaclava on the head, and emergency bivy...not bad for 4+ ounces.
I've not used it since 'evolving' but now that the subject has come up I might dig it out of the reserve kit pile.