Extremely thought-provoking (in MY head at least!) thread, Mike.
I find myself disagreeing with TEP however, In that I believe the single most important driver of 'The Vernacular' is actually the 'Ready Availability of Locally-Sourced Materials'.
Viewed in this context, Vernacular Architecture is really the fore-runner of, yes, you guessed it, this year's Buzz-word: Sustainability.
Thus, while certainly taken in the context of the local climate, a local workforce with apprenticed-skills (learned from their fathers and their fathers before them), would use material in a truly LOCAL context.
In GCA terms, to me this translates as an Architect properly analysing the climatic, geographic and botanical (to name only a few) constraints on a potential design and then properly applying that analysis in execution.
For example, in any landscape-design work, I certainly always try to utilise Native Species in preference to 'Exotics'.
FBD.