Here is some data (okay, gratuitous nerdery) relevant to this question.
First re "The End of the Story": this is the
first-composed of the Averoigne stories. Smith reserved the right to fine-tune his geography.
"The End of the Story" sites only Perigon and Faussesflammes in relation to sites of historic AD 18th century France specifically the Tours-Moulins road. Vyones is mentioned only in a quoted document, as a city existent in its (parallel?) past; other sites – Ximes, mainly – remain absent.
I mostly want to discuss the career of the Languedoc / Languedoïl dialects in the hinterland. Because the shift in their boundary – favouring Middle French in the late 1300s – overlaps the earlier stories.
The Albigensian crusade in the early 1200s weakened the Languedoc relative to Paris. Other shocks, particularly the 100 Years War, allowed increased Parisian control over the Auvergne region and more prestige for langue d’oïl here. In particular: Good King John (Valois) recognised John the Magnificent as overlord and duke, and so the latter ruled 1357-1416 over both Berry
and Auvergne.
So if Averoigne (whichever it is) wasn’t speaking some Old French “Averoignais†before 1357, it was certainly speaking standard Middle French by 1416.
Since the quoted document in "The End of the Story" is in fact Old French, those events precede 1357. Vyones had its cathedral 1138.
So far, we can be assured Vyones was Languedoïl as of 1300. Besides this, and before that, though…
Of all the Averoigne stories, by Smith or by others, only “Maker of Gargoyles†and then “Holiness of Azédarac†(plus “Doomâ€) specifically postdate the Romans and precede the Albigensian crusade. (We’re not counting “Cult†in between.) These two are the only stories which I would accept into evidence for the Averoignais / Averonhat dialect of the twelfth century and before. Also they too are early-composed. And they don’t say much about the language; Vyones is likely polyglot anyway at this time.
Here we have to account for Glenn Rahman’s “
The History of Averoigneâ€. This has Lovecraft assuming Auvergne for the later stories and
not getting corrected by Smith.
As mentioned I do like the Berry theory… for Vyones.
For Ximes, Smith doesn’t say much about the town proper until “Beastâ€, set in 1369, at which point in our timeline the Auvergne
is (politically) Berry, and all the educated men are speaking Middle French.