I can totally understand your confusion, as recommendations of the "right " way to approach an issue, has to be taken in the context of the building and it's environment.

Your comments above are, I believe, generally correct in that the attic zone is very often a dehumidifier for the house. That is why some (even recently built) dwellings do not have a VCL across the ceiling-to-attic interface. That's not to say it should not be air-tightened.

If you delete that vapour removal zone, (by adding a VCL), then you will be changing the dynamics of the building. Make those changes without considering the impact at your peril. A lot of homes that have draft proofed and insulated with best intentions over the years, have suffered.

Understand what the result of your changes will be, then consider how to mitigate. Add a VCL to the ceiling - what do you do to replace that moisture removal. IWI and seal up all leakage paths - again what is the consequence and how do you mitigate.

Point is, there is no one right answer. You need to take a wholistic view of what you are trying to achieve, and what the various possible actions will give rise to. I've seen VCL's in houses built from say 1970's, but the totally inadequate insulation and heating meant they "sweated", rotted, moulded. The intent may have been right, but there was no understanding of the consequences of the addition of this layer.

Worse than all that, is that there are various opinions on breathability of say wall, and often those calling themselves experts are 180o opposed in their views, plus those with an agenda to sell their products.

So, is adding a VCL right...YES and NO. It's entirely case specific.

"...Are old and new roofs principally the same?.." in as much as they shed water yes, however one huge factor is the vapour permeability of the skin, inside to outside. Old roofs may have felt (totally impervious to vapour), or they may have no felt. New roofs may have very vapour open membranes, or less so. Old roofs built with lots of eaves to ridge ventilation may have had that altered by enthusiastic DIY'ers, getting rid of the "terrible drafts up there".

Or maybe they never had that ventilation in the first place - less of an issue 100 years ago, bathing once a week, and open fires. New houses can be designed or built badly, so they don't perform as they ought to.

This has completely NOT answered your question, as there is no one answer. You need to understand the particular case. Then chose a path that delivers your objectives, without detrimental effects.

I have a little mantra, which does not state the "right" answer, but establishes a thought process...

"If you insulate you should air tight. If you air tight you should ventilate. If you ventilate you should recover the heat". Essentially, maintaining a balance.

Not wishing to hijack this thread but I have some related queries that I’ve been pondering and instead of starting a new thread I thought I’d tack on here:

In the case of the addition of a new warm flat roof to the loft space, where most of the guidance I see is to have a VCL above the deck / below the insulation, how do you join that to the VCLs of the adjoining walls?

Posted By: tonyFor me v/b on all ceilings, I make it the air tightness layer too.

One approach is, as you suggest Tony, to have the a/t layer at the ceiling, then the joins to the walls are fairly straightforward. But does that then mean that, for example in a warm flat roof construction, you would end up with two VLCs? One on the inside and one on the outside?

What about if you want to leave your ceiling joists exposed, so therefore no internal ‘ceiling’ to speak of, then what alternative might there be?

Apologies if this has deviated too far from original thread

"What about if you want to leave your ceiling joists exposed"

Painstakingly cut the VCL and seal it to the seams so they pass through it