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assuming that a VBL is usefull when ever the temps will be below freezing,how many nights do you think you could spend in a down bag without a VBL in below freezing temps before it lost it's insulating capacity?Those VBL's look like a good idea, but it seems like it would feel as though you were sleeping in a trash bag

That is going to be incredibly hard to quantify. It depends a lot on your individual metabolism, outside temperature, your sleeping bag material, etc. I personally wouldn't worry about it in reasonable cold (call it above 0) for a weekend. If I was going out for a week or more in very cold temps I'd consider a VBL.
Some people actually use trash bags, so I guess that'd be true.