It is possible to get VBL clothing items that can go under your down jacket and down pants if you want to do that.
If you buy the stuff from the pro-makers of VBL clothing, it's going to cost a pretty large amount of money. The VBL sacks are much more affordable, and lighter.
An effective VBL that is properly operated by the wearer can give maybe 10-15 degrees more warmth than without the VBL. But this is anecdotal and variable, and must be tested by the user to see what they really can get out of it in the real world, with their gear set-up. Try it under controlled conditions where you can add warm insulation to your situation if the VBL is not giving the expected results.
Have a thermometer and some insulation layers available during the testing, so you can record what is happening at various temps, and this will become your reference data for future use.
The general benefits are that the down insulation retains all its loft, and is not degraded by body-generated moisture penetrating the insulation layer.
There is essentially no evaporative cooling happening at the skin level once the humid micro-climate is established inside the VBL, so you feel warm.
You have to be good at venting and layering, in order to properly operate a VBL.
If you are sweating in it, you are not operating it properly. But, it will have some moisture/humidity in it as part of its normal behavior. The baselayer inside the VBL is to keep the system from feeling "clammy" on your body. The insulation layers outside the VBL are used to maintain the neutral comfort temp. You must strive to maintain a neutral comfort temperature which is warm enough, yet not sweaty. It takes some practice, because it's typically warmer when you go to bed, than it is in the pre-dawn hours, and adjusting insulation is critical to maintaining this neutral comfort temp.
When you get up in the morning, it's going to be moist inside your VBL. This is normal, because that is part of what makes it work.
So, you should slowly vent it from the hood or openings, to allow some moisture to escape as humidity out the openings, without letting a huge blast of cold air in all at once. It will be quite chilly if you just expose your moist body and baselayer to cold air in the morning. So, vent some humidity off gradually, before getting out of bed. There will still be some moisture in there, but try to minimize it. I towel off right away, and get some wicking layers on over the top of me.