Just to show how different individuals can be, I always use vapor barrier clothing in my sleeping bag when it's below freezing. I have non-breathable rain gear and wear it over a base layer. If it's really cold (like my recent 18*F night) and I need more wraps, I put them on over the rain jacket so they won't get damp.

The theory is that once the humidity inside the vapor barrier gets high enough, your body stops producing moisture so you stay in equilibrium. It does work for me, and keeps my sleeping bag a lot drier. Without the vapor barrier, the moisture from my body condenses (and freezes) on the underside of the outer shell (I could feel it there!). A couple of nights of that can really reduce the effectiveness of your bag. Remember, though, this is for well-below-freezing conditions, not for the low 30's or higher.

I wouldn't ever want to try a vapor barrier when I'm active, though, which is where I part company with the Stephenson folks. I try really hard not to sweat when I'm active in cold weather.