Absent audio recordings, I'm somewhat at a loss as to how to "prove" ordinary people were still speaking Latin under Vespasian, except that people usually write graffiti in the language they use in conversation, and as far as I know, Latin graffiti, as well as informational inscriptions for everyday matters, outnumber those in other languages in Rome and Pompeii. I'm quite sure you're better informed about Italic languages than I am, but I was under the impression that evidence for these dries up before the later Imperial era.

There also seems to be some doubt as to whether Rome was ever the capital of the Empire, or Latin ever used as the official language of Imperial Rome. Just so you know what sort of sources might be relevant. Please understand that I'm presenting questions brought up on the talk page, for which I have been unable to offer answers that satisfy other participants.