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First, it's Ventum not "Ventrum".
Second, Richard Nisley tested Ventum at 6.6 CFM: https://backpackinglight.com/forums/topic/100720/#post-3495273. His measurements are usually taken to be reliable so I'm not sure where the confusion is coming from? Is there some other claimed measured CFM for this fabric?
Third, non-air-permeable membranes do *not* rely on pressure differentials. If there were a pressure differential your jacket or sleeping bag would puff up around you like a balloon. They rely on humidity differential. If it's humid outside your jacket/bag then they will work less well than if it's very dry outside.
Finally, since air-permeable fabrics rely on air-movement if you are inside a relatively still tent the fact that they are air-permeable is not going to matter much. Most of the moisture exchange is going to be due to the humidity differential.
People have long used bivy bags and sleeping bags shells made out of eVent and similar fabrics with FAR lower air-permeability than Ventum. Western Mountaineering use Windstopper on their higher-end sleeping bags which is not at all air-permeable. There is no reason to think that you need a high air-permeability fabric on a sleeping bag in most situations.
Of course more breathability is nice but if the climate is already very humid then no fabric is going to "breathe" very well, since there is nowhere for the excess moisture in your bag to go. If the climate is fairly dry (or even moderately dry) then all fabrics will breathe better and anything with higher breathability than Gore-Tex is going to be just fine. This is of course conditional on you being able to regulate your temperature reasonably well. If you are really overheating nothing will prevent from waking up soaked in sweat. Also, more breathability means less water-resistance.