Vapor barriers for sleeping bags are a separate thing - in very cold temps, you don't want your own moisture (you give off about 12 - 14 oz of moisture from your skin, excluding sweat) to get into the insulation. I have an underquilt with a mylar liner for subzero temps.

Emergency bags are just that, for emergencies, because you will be anything but comfortable with them. You'll survive, but sleeping in one is ... well, I'd have to be completely exhausted to manage it.