I wear a thin layer of merino wool--prefer Patagonia wool 2, but the 150-190 weight Icebreaker is good, too. I've noticed no difference in speed of drying between thin wool vs. synthetic layers when I keep either of them on.
As far as Hotsac vs MB parka, not quite a "fair" comparison. Remember, one significant advantage of the VBL is preventing evaporative heat loss--and in so doing, keeping your bag dry. The number one way of getting a bag wet is sweating in it... So for boosting your temp rating and overall "bag maintenance," I'd go with the VBL. Now, you'd still want a poofy down parka for around camp, and you could layer that blanket-style between the Hotsac and sleeping bag if needed. VBL clothing does have the mentioned advantage of being able to layer over w/worn clothing...
Oh, and to second the "when to use" comment, I bring the VBL on any trip expected to be freezing or lower, FWIW.