Use VBL @ 35 degrees in 40-degree bag, or bring warmer bag?

Personally, I think it might be a rather bad idea. You would sweat badly inside it. Just take a little extra clothing and wear it if necessary. Cheers

Roger, thanks for responding. What's your cut-off point for using a VBL? Max temp?

VBLs work at any temperature. I have a MYOG cuben summer bag and a MYOG 3 season cuben bag. So I always use a VBL bag, even in summer. "Sweating" is not a problem in a VBL any more than it is in any other bag; if the word sweating means visible perspiring due to being too warm. If someone is "sweating" in a sleeping bag the bag is too warm for conditions or the bag needs to be unzipped. Insensible perspiration does condense inside a VBL. When I get up in the morning small drops are noticeable in the foot area. I just wipe them off. Not a problem, and I don't have any sense of feeling clammy. It can feel somewhat humid, but not unpleasantly so. In fact, the humidity makes the warmth comfortable.

Backpacking Light’s Andrew Skurka on Vapor Barrier Liners: What they are, how they work, and when to use them

Peter's statements match my experience also. In a sleeping bag with an added slip-in VBL, you can vary the warmth by rolling down the VBL–to reduce the warmth, and vice versa, along with zipping open the bag as needed. I've been using a 35F rated bag + VBL for many years to cover a wide temperature range (teens to 50's). A neck gaiter and extra wool or fleece hat (along with extra clothing+VBL) can greatly help when you drop into the lower temperature ranges. Practice this a few times–you might like the advantages.

I use a VBL with a 30 degree quilt, why is a long story I don't want to go into now. However, I find it to be warm and have not encountered any problems with condensation that caused any discomfort. If it gets too warm, venting is simple enough. I have used the quilt/VBL combo from the mid 40's to the upper 20's.

One or two nights and you're OK without a VBL as not enough perspiration will build up in your bag to notice – especially if you can air your bag out during the day for at least 30 minutes.. Beyond that, yes, definitely use a good VBL inner bag. (Ever read about the disastrous Scott south pole expedition? Their down bags were so frozen solid ager many days ion the trail that it often took an hour to "melt" their way back inside them.)

At 35 degrees I think your perspiration should evaporate readily leaving you with a dry bag in the morning… I think. Would high humidity change this?

yeah, has to be below 20 F or so. Then, the point where it goes below freezing is inside your sleeping bag. There's also a 10 F drop in the inch of air space outside your sleeping bag. If the point where it goes below freezing is inside your bag, then sweat that's evaporating out of the bag will freeze at that point, and then evaporate very slowly. So your bag accumulates ice over a few days. Like Skurka said, many nights under 20 F or so. You can use a VBL at warmer temps and shorter number of days. It makes you a little warmer. It would weigh less to have a little more down in your bag.