It's quite common, particularly in cold weather sleeping situations.
The "wicking" part is your baselayer long underwear.
The "Vapor Barrier Layer" is usually a bag that is waterproof, and non-breathable, and can fit in the sleeping bag with you.
Your body and baselayer inside the VBL creates a "micro climate" which keeps you warmer and stops any body moisture from penetrating into the down insulation of the bag, keeping it dry and able to fully loft for better insulation performance.
If you want to shield the down insulation from external moisture, you can then put a waterproof bag around your down sleeping bag on the outside, without fear of wetting out the bag from your body moisture inside, because you are using the VBL.
There is actually a company which makes VBL clothing with a fabric flocking inside the non-breathable VBL layer, and it looks like long underwear, and has the advantage of not being restrictive of your arms and legs. It wears just like long underwear.
It's quite expensive, and heavy.
Your cheapest and easiest bet to get to where you are going, is to use a good wicking baselayer, and use an Adventure Medical Kit SOL emergency bivy bag that costs $16 and weighs about 3.5 ounces as your VBL bag.
You have to get a "feel" for using this VBL concept because it is so much warmer that you can easily sweat too much inside the VBL.
So, you have to pay attention to your body temp, and be able to layer off insulation or layer on insulation, depending on the need.
After a while, you get to know what is going to be needed in various situations, and it becomes a routine activity to dress for your need for the temps and VBL.
Like anything, it requires a little practice to get to where it feels like "2nd nature".
Regarding multi-day outings, that is where this system actually shines the most, because it keeps the down from accumulating moisture over several days time which can lead to insulation collapse. And insulation collapse can be dangerous.
The VBL is much easier to dry out on a daily basis because all the moisture is on the surface inside the VBL, or in the wicking baselayer, both of which dry quickly.