I agree with Tod, there isn't enough air pressure generated by the person in a sleeping bag to effect condensation. Generally fabrics are pretty good at blocking air. Pertex, eVent, Gortex, etc form a membrane to prevent air displacemet. Down or synthetics are designed to stop air exchanges. Not so much to prevent condensation, but to keep you warm. Conduction and convection are the big heat losses inside of a bag. IR is only about 10-15% of your heat loss. You are saying that a permiable fabric will cause condensation. This depends on conditions. Yes I agree it can since it depends on a differential (usually just heat and humidity.)
IFF our fabrics were perfect insulators, there would be no condensation. You need a heat differential to form a dew point. Water would evaporate to 100% humidity but not evaporate any further given a constant temp. You would still sweat (insensible perspiration) and you get clammy, damp and sticky... generally a VB can simulate these conditions by stopping water and vapour transfer. You change the micro-climate next to you.
IFF you add any permiability for air escape, the normal condition, then water vapour and heat will escape with it. Now there is a problem with condensation.
Since temperature is not a constant, nor is humidity, nor wind pressure (providing far more air pressure than your body heat,) it doesn't really help determining how to make a bag to utilize this. We have vapour barriers with their problems. We have all the rest. How do you propose to solve this problem?