> If you put your insulating jacket on top of your shell jacket, your shell layer (being the greatest barrier to vapour transfer) is kept warm and efficient, making it easier to transfer moisture to the next layer = insulating layer which should be much more breathable than your shell layer and thus showing little resistance to moisture transfer to the outside.
Unfortunately, this logic contains one fatal flaw. And 'fatal' could be the literal result in some cases.
Yes, the WP/B layer may stay warm and dry, and transmit your body moisture to the next outer insulating layer - at the start. I have to ask what purpose the WP/B layer is now serving - none at all as far as I can see.
However, assuming the outside world is seriously cold, then the freezing point must be somewhere inside the insulating layer, between your warm WP/B layer and the outside world. This is where you will get a build-up of ice: INSIDE your insulating layer. Make no mistake: ice will form there. The WP/B layer has done NOTHING to stop this.
It gets worse. As your body temp and heat output fluctuate, so the position of the ice-forming layer will vary back and forth, eventually resulting in a thick frozen layer inside your 'insulating' layer. Needless to say, the insulating properties will die, and you will be left freezing to death.
If you are going to wear a VBL, you must make sure it really does block ALL the vapour, so that you don't get a build-up of ice in the layers outside it. And remember: WP/Breathable is NOT a Vapour Barrier Layer. One breathes; the other does not. Ideally, while wearing a VBL you should ideally make sure that you are cool, not warm and sweating.
Basically, a VBL layer inside a sleeping bag is fine, but a VBL while you are actively walking is tricky. If Mark Twight wants to wear one while doing 'extreme alpinism', good luck to him. But he probably spends a lot of time standing still, belaying, and this is very different from active walking with a pack.
(Exception: VBL socks are in a class of their own. They can actually serve a slightly different function, depending on the outside temperature.)
So what use is a WP/B layer when walking in sub-zero conditions? Well, as an outer shell it blocks the wind ... and this is useful of course.
One useful technique, IF you can manage it, is to try to have the ice build-up on the inside surface of a thin shell layer, so that you can take it off and 'brush' the ice layer off it. In other words, try to control where the ice is going to form, in a place from where it can be removed. Not easy though.