A vapor barrier liner in the form of a large garbage bag or two, welded, glued, or as a last resort duck taped together would add up to 10 degrees to your sleeping system, probably replacing the need for a wool blanket, and unless your sleeping bag is huge, the wool blankets won't fit inside or if put on top will compress your insulation. A second sleeping bag or some sort of quilt placed on top of your sleeping bag would be ideal.
You should have at least 2" of closed cell foam for a sleeping pad on top of snow.
Digging a proper snow cave or quinzee, blocking the door, and sleeping in it would create a room that remains at least at 32 degrees, reducing your insulation requirements considerably. You'll need snow shovels and maybe a Frisbee or two and wear ponchos or rain gear while building it because otherwise you could get soaked. http://www.princeton.edu/~oa/winter/wintshel.shtml
http://www.gottagoitsnows.com/snowcamping/007.html
http://wildwoodsurvival.com/survival/shelter/snow/index.html