In extremely cold, dry conditions, vapor barriers can be part of an effective clothing or sleep system. According to the book Secrets of Warmth by Hal Wiess, the human body gives off moisture for three reasons: (1) as a fear response, (2) as sweat to cool us down when overheating, (3) to maintain adequate humidity (~79%?) for the skin. There is some debate in the backpacking community as to whether the insensible perspiration (aka transepidermal water loss) is directly related to keeping the skin at a fixed humidity… but there should be no question that the micro climate near the skin effects rate of transepidermal water loss. This is alluded to in numerous scientific articles such as Eero Lehmuskallio’s thesis Cold Protecting Emollients and Frostbite. [I will do a more complete literate search later… most of the article I found were about care for premature babies.].
The most important reason to use a vapor barrier is to keep moisture out of your insulation. If you are out for more than a couple of days when the conditions are constantly below freezing, moisture from your body will condense and then freeze in your insulation. This will result in your insulating jacket getting heavier, stiffer, and be less insulating. A vapor barrier prevents this from happening.
A vapor barrier is likely to make you feel warmer, and you will use less energy because the vapor barrier trap some moisture near your skin so you don’t need to perspire as much. BPL has a brief article on vapor barrier liners. There is a nice Introduction to Vapor Barriers by Andrew Skurka. One of the early modern proponents of vapor barriers is WarmLite’s Benefits of Vapor Barriers. You can see a rebuttal of some of the warmlite VB claims.
I found vapor barriers useful on some extremely cold trips in northern Canada. When I was active, I found that vapor barriers didn’t seem to help me until the temperature was below 10F. I found that vapor barrier socks are great, pants and mittens are fair, and that shirts were useful but hard to get right.
I found when I was inactive or sleeping vapor barriers worked very well when the temperature was 20F or lower and I was using appropriate amounts of insulation (e.g. don’t overheat or you start sweating and it doesn’t have anywhere to go).
Very few companies make vapor barrier clothing. I would recommend checking out rhdesigns and warmlite. The cheapest way to try out vapor barriers is wear light polypro liners, with sandwich bags as a vapor barrier followed by wool socks (your insulation), try some latex surgery gloves over your hands and insulate your insulated gloves or mittens, and/or a plastic trashbag around your torso. If this works for you, take a look are some of the better quality materials. These days I am not doing extended winter trips and am not using vapor barriers.