The one key thing about VBL's that you need to remember is that they are designed to prevent the loss of heat through "insensible" perspiration - that is, the perspiration that goes on all the time to keep your skin moist, as opposed to "sweating" or sensible perspiration, which is intended to cool you off. If you ar using VBL clothing and you are warm enough to "sweat", then you either have too much inslution over the VBL or the temperature is simply too warm for you to use it. For use during strenuous physical activity, it's tricky to keep that balance, and it has to be [Edited for inappropriate languge, please review forum policies for more information] cold if you are really exerting yourself. If you can maintain the balance, and not "sweat", then it can work well. But it seems that not many folks do use VBL clothing during strenuos activity (except for socks). It's much easier to maintain the proper balance of insulation while you are sedentary (especially sleeping), so that's why a lot more folks successfully use VB sleeping bag liners. But VBL clothing can be very useful for sleeping - as it's much easier to adjust your insulation by unzipping part of your sleeping bag, for instance if your wearing VBL clothing than if you are in a VB liner. Plus you can get up to pee without venting your VBL, and without risking a chill from the evaporation of moisture from the base layer you are likely wearing under the VBL.