I've never used a VBL bag liner, and the idea doesn't appeal --- but then the idea of water vapor freezing in among the down in my bag appeals even less. I just don't get into temps that require it often enough to have ever used one.

I own VBL socks, and have used the poor man's version of those a time or two as well (bread bags or the like). Ditto for handwear. That's about it for me.