Sweat and breath moisture gradually accumulates in sleeping bags over several days, regardless of insulation type. In winter this problem is exacerbated many times because bags can't be easily air dried.
This means for near and below freezing temparatures a VBL is essential to retain bag warmth AND reduce carried weight. And, yes, ya gotta wear some light poly long johns to bed inside a VBL to avoid clamminess and direct conductive heat loss.
The ill-fated Scott south pole expedition might have had some men that made it had their down "sleeping robes" been used with a VBL of some sort. Often the men had to take an hour or more just to get inside their frozen down bags.
So, yeah, for week long or more winter trip ya NEED a decent VBL. If it isn't comfortable (too clammy) "deal with it" OR if you feel too cool you likely need a warmer bag or mattress. You'll easily know which it is, of course.
I'm finally making mine this year from waterproof aluminized ripstop I got from Seattle Fabrics a while back. MUCH cheaper than ready-made VBLs and now I won't have to spend hours airing my bag before bed time and still not getting it completely dry and then still having to wear VBL clothing in the bag. VBL clothing in a bag ain't 100% efective unless ya duct tape it closed at ankles, waist and wrists!
This isn't about whether BVL "works" for you or not. That's a subjective issue. It's about keeping the d@mn vapor from accumulating in your bag and thus keeping your insulation dry and effective day afer day. PERIOD.
"There are no comfortable backpacks. Some are just less uncomfortable than others."