It seems wrong to me for the down to be anything but the outermost insulation layer.

I just thought of how I might use a vapor barrier layer. As I said, my preference is to sleep near naked in conditions above a bags rating, and to wear dry wool underwear or a dry wool liner inside the bag when well below a bags rating, but never to the point of sweating. The wool captures the water vapour, and the insensible heat of the water vapour, that your body gives off even when not sweating. There is always some. You need to dry the wool out for the next cold night though. That's the catch. The vapour barrier would be handy if you get caught on a night of extreme cold conditions where for whatever reason your wool is already wet. So you could wear the wool under the vapour barrier in those conditions, to stay warm, but protect the bag also. I think my wind layers might be good enough though, for that remote possibility.

We can get -30F, there is a chance of that, but there's no way I'm buying a -30F bag for that. I'm not saying what rating it needs to be cause I'm not exactly. I've down -30F in the backyard in my 3 pound 20F synthetic bag, but that was just one night and not a true test and I can't remember what clothes I have. I will test it out some more this winter though, then finally get the winter bag I figure out I really need for around here. I'm thinking still 3 pounds, but good quality down.