Back in November/December I did a few 4-5 day camp outs- bow-hunting.
I noticed I'd sleep colder after 3ish nights especially if it was rainy/damp and overcast days. Night time temps- teens- 20's.
After that happened- next time out I took an Equinox nylon bivy and used it inside the bag. It helped but it's somewhat breathable so it allowed some body moisture into the bag.

Western Mountaineering sells a VBL- it's a mummy shape which I don't like. Not worrying about drying the bag out would be nice.

VBL's really aren't necessary above 0*F in my experience. How are you sleeping out? In a tent?

Yes a Shires Tarptent.

Hi Mike - which model? Single or Double wall?

Single Wall, Contrail- lots of condensation .

That's why I was asking. Your problem is your tent, not your bag. You really don't need a VBL in the temps you describe, but what you should be thinking about in wet/humid conditions is a double wall tent.

Yep, I'm going to get a four season tent.

Down bag?

20+ years ago did a 16 day DIY float trip in Alaska for moose/caribou. One stretch it rained for 6 days straight. By the fourth night I started getting a little chilly although temps never got below high 30’s. My WM down bag with a Gore Tex shell was becoming damp. Even though we tried to be reasonably careful about moisture management in the double wall tent it was 100% humidity nonstop, plus our bodies are always releasing moisture. The down wasn’t getting wet from the outside, it was getting wet from the inside.

Waterproof/water resistant-breathable shells are made so vapor passes through but not water molecules. But when it’s a 100% humidity outside the bag the vapor passage slows down considerably and will begin forming into water molecules, hence trapping itself in the insulation.

Two years later we went back on a repeat hunt trip and that time I brought a synthetic bag. It rained more the second trip but I had no problems with a damp bag.

Loved my WM Antelope for backcountry winter ski trips in the San Juans when we saw temps 20 below.

Today I use a KUIU treated down bag. I know Brad’s feelings on this but at the very least treated down dries NOTICEABLY faster than untreated down, and does not seem to get as damp as quickly compared to untreated.

Goretex was one of the worst ideas ever to be integrated with down bags, and of course Gore ultimately developed the far more breathable Dryloft for down garments and sleeping bags.

Casey,

Yes Down bag- a WM Mitylite with a Mountain Laurel Design quilt type bag over it. I read here about using that combination and it has worked pretty good. But the damp overcast days/nights of rainy weather make for cold sleeping after a few days. Even my Down Coat loses performance.
That's another item I'm in need of- a lite-weight synthetic coat.

I have become a huge fan of Primaloft for outerwear. Closest thing to down without the drawbacks. I have been waiting to see if will be successfully adapted to sleeping bags.

Even the other breathable-water resistant outer shells inhibit passage of vapor in high humidity conditions—although I agree about GT as a bag outer shell!
The characteristics of down doesn’t lend itself to allowing vapor to pass through either.

VBLs work great IME. But most, or at least the ID I have, are kind of cramped. As a result I avoid using it whenever possible. Only on week+ trips, with well below freezing day time highs expected. Think -20C to -30C, with no hope of thawing and drying your sleep system.

I’d recommend trying additional external layers, like a light quilt, to move the freezing point of your body’s condensation further out of your sleep system, hopefully fully out of your main bag. A synthetic EE guilt would be ideal for that, but even a cheap Costco down quilt helps.

In my experience vapor barrier liners work well but some people just can't get used to them.