Interesting- if a trip incorporated stops in huts or cabins, this certainly would test the limits of a system where VB and insulation could not be separated.
Some ideas that have been put forward so far about incorporating clothing as insulation with a VB liner in an SB include include arranging items like jackets and vests outside the liner, putting the foot end of a VB inside a jacket or vest, and wearing the liner up to just below the armpits, therefore allowing for the option of wearing a jacket or vest while sleeping. It would seem this option would compromise, to some extent, the main goal of the liner, which is to prevent internal moisture from getting into the SB. While I've not experimented at all with heavier layers inside the liner, from what I've read it seems less effective- more layers to get wet, less chance of establishing that elusive VB microclimate around the skin, etc. VB clothing would seem to be more flexible in this regard, and yet others have said they still prefer a liner. No experience with VB clothing here.
I'll share the results of last night's backyard VB test session in low temps around 2F, cold enough to render an msr windpro chilled to air temps intert even with an inverted canister. Yes I know most stay inside in this weather, but I prefer to see my behavior as motivated by a passion for scientific knowledge (thanks to Roger C.) rather than an indication of deeper psychological unrest.
While I know many use VB liners effectively, and in much more extreme temps, my experience may be of interest to VB novices interested in extending the range of their sleep system and experimenting before making larger investments in VB gear. My MYOG VB liner consists of 3 hefty ultra flex large trash bags (30 gal, 113 l) taped end to end, with a doubled-over section near the head through which a line was run with a toggle for tightening- the toggle was operable from the inside, and this helped tremendously.
I slept in an mld soul bivy, on top of plenty of ground paddage, using an rei sub kilo 20F down bag, later complemented by an mh phantom down bag. Inside the liner I wore merino midweight layers, including gloves and liner socks. On my head a neckwarmer and warm fleece hat, underneath which I wore a buff (those tubular things).
First, the experience of a VB in temps from 20-25F versus 0-5F was a very different experience. While the higher temps made the VB, for me, clammy and unpleasant, I never experienced much moisture in the VB liner last night. The ability to effectively cinch the top of the VB (from the inside) helped keep it in place and seal warm air in. I stuffed the mh phantom SB into my bivy in the middle of the night to help with cold feet, but in retrospect stuffing the end of the liner in an insulated vest probably would have sufficed, and I did eventually overheat with the phantom in place.
While the full VB liner arranged up to the head regulates almost all interior moisture, the main disadvantage is that one's arms are inside the bag, making it impossible to adjust anything outside the liner while inside it. Taking steps to make sure your pillow or additional pieces of insulation don't move much in the night will make things more comfortable. With the liner up to the armpits, arms are free, and one can wear a jacket or vest on their body with the VB in place. In practice, with the liner in this position, a small amount of frost did form on the inside bivy surface above my upper body, most likely moisture from arms, chest, etc. Farther down towards the legs, everything stayed perfectly dry.
Roger wondered about the durability of the trashbag VB liner. I've seen no indications of wear, though the liner does include dust tape at certain stress points. I'm sure, though, that the primary merits of the trashbag liner are its easy repair, and its minimal expense.
As I get better at using the liner, I may find it easier to use it in full length form. Wearing it 3/4 length allows use of arms, yet also compromises moisture management to some degree. There is always the option of starting the evening at 3/4 length, and drawing it full length as things chill down.
Anyhow, I enjoyed the experiment, and would be eager to hear from others who have developed useful tricks with their VB liners. I am still trying to figure out how best to deal with frost from respiration forming around the head of the bag- breathing through material does seem to help. I am also investigating ways to fix in place the lip of my bivy so that it stays close to the breathing hole of the SB so that most condensation occurs on the bivy itself.