For best breathability in the very low temps, my canvas winter sock is great. With using VB clothing at night, and making all effort to keep the down dry, the last thing I want is to trap water vapor inside my hammock sock or rain down frost trying to get out. Obviously the canvas comes at a significant weight penalty, so I only use it when I'm also pulling a sled.
I have not tried experimenting with a frost bib to help capture some of the moisture from breathing, that may allow for use of the lighter but less breathable fabrics. In the cold I tend to stick with what I know will work.
I have used a simple nylon sock in cool weather when more ample venting is easier and cold protection not as critical. Though, I would like the option of a packable cold weather sock for times/areas when it would be best to leave the sled at home.