I'm not big on VBLs. I know they work in very cold weather, but I've not had any problems in Yosemite at around 15F at the coldest. I don't seem to sweat that much and am not gone so long that my bag gets soaked, but for a longer trip in colder weather, they may be a good idea.

Heather-are you talking about these things-
http://www.armysurpluswarehouse.com/black-mickey-mouse-boot-with-valve.html
I saw this link on another site I belong to.

Here's a bunch on eBay-
http://www.ebay.com/sch/i.html?_nkw=extreme+cold+weather+boots