It requires a little bit of practice with your technique.
The VBL needs to go over your base layer. This creates your "micro-climate" where the baselayer holds the moisture inside the VBL sack, so that it doesn't feel too "clammy" when you are in the bag. There will be some moisture in there, because that is part of the method.
The key is to not turn it into a sweat-box.
And to get a handle on this, you need to be good at venting and layering. If you feel too warm, then shed a layer, or vent something open, to keep you at a neutral temperature. Otherwise you will start sweating, and that is what you want to avoid.
If you begin to feel chilly, add a layer or zip up the bag, or whatever it takes to get back up to neutral comfort level, but NOT hot.
The goal is to avoid extremes, and stay in the neutral comfort temp zone.
After a while, you get accustomed to what it takes to keep your body in a neutral temp state with this VBL on. Then it becomes very easy to manage.
Like many things, it takes a learning curve to get good with it.
But it does work.
I think the WM Hot Sac is too expensive for what it is, and I have heard that it gets the silvery color onto your baselayer. I haven't used it, but some have mentioned the silvery stuff coming off onto them.
I use the $16 Adventure Medical Kit SOL Emergency Bivy bag that is about 3.5 ounces, and works real good inside the sleeping bag.
It doesn't have a draw-string, but you can just gather it up around your neck, and it works fine. If you want to add the VBL to your head(which helps a lot) then get one of those cheap women's vinyl shower caps that "balloon" out to cover the hair-do, and has elastic sewed on the bottom to keep it on your head. Very light and works great. I use one.
And remember, once you are using this VBL inside the sleeping bag, then you can use a waterproof non-breathable outer bivy covering as well, and it won't wet out the bag from the inside. I use another AMK SOL Emergency Bivy for that outside coverage purpose too. This protects the down bag from getting moisture from the outside rain or splash or whatever, and the VBL sack inside the bag protects it from any inner moisture build-up.
But you have to watch that you don't get too hot, or overcompensate with the layering if you get off the neutral temp. You can get too hot very easily, and if you get cold, you tend to over-do the warming-up part.
Practice makes it work best, and you learn how your body reacts in these situations, and you learn to make it work for you.