I need to actually agree with Jeff here. Not quite sure what a vapor barrier is, or if it goes over or inside of the sleeping bag?

I was thinking of doing something similar to the US system: I have an REI sub-kilo bag (20 F rated) and a (leaky) bivy bag. Rather than drop a few hundred on a system, could I just buy a cheapie as the second bag? Something like:

http://www.sportsmansguide.com/net/cb/cb.aspx?a=245534 OR
http://www.sportsmansguide.com/net/cb/cb.aspx?a=78532

yeah, they're heavy, but they're definetely a weather-specific item. the other option would be to buy a fleece sleeping bag and put that inside of the REI bag.

Ideas on which would be more "bang for the buck?" I don't camp out as often as I'd like, so I'm leaning towards the cheaper/lighter fleece. But... would that really increase the temp span that much?