Andrew the idea as I've understood it and practiced it is to keep your insulation layer dry. If the vapor passes through the gtx sock it'll condense in the next layer which is the "real" sock and also the insulation layer. Plus there's the "push-pull" aspect of gtx so a lot of the vapor might work its way through.
I guess David's system is using the neoprene sock as the vapor barrier and it also provides insulation as well! Looks like it ought to be bomb-proof for really cold weather or deep snow. (they come together???)
For less extreme conditions I wear a very thin liner, then the bag, then a sock and a gtx shoe but don't bother with the liner unless it's at least freezing.
usually the sock comes out at the end of the day in very good shape. If you can find the little 3 or 4 gallon scented trash bags it'll even smell great and why not….hasn't been against your sweaty foot or the outside world. Plus it's pretty easy to rinse and dry a thin liner so you might get a couple days use with the same pair of socks and the socks can also be your camp/sleeping socks.
What're everyone's thoughts/experiences with gtx or non gtx shoes in completely frozen weather?