I don't really understand what /u/gordiep is trying to say.

Naturally, traces of the vulgar register are bound to be found in the literary language in certain contexts - e.g. when describing everyday life in a satire. That doesn't make them any less vulgar than e.g. Brian Jacques letting some of his characters speak with a Scottish accent (to mention an example that caused me not insignificant trouble when I was a pupil learning English) means Scottish and literary English are one and the same.

I could imagine that our differences might mostly come down to semantics when talking solely about the 1st century BCE to 2nd century CE but I don't really see how one could deny the existence of a growing divide between vulgar and literary Latin from the 3rd century onwards, it seems so obvious that Late Latin authors are not writing in the common language of their day - which imo wouldn't have seemed strange to anyone at the time because it's what the Greeks had been doing all along. In fact I think the Greeks are a good point of comparison for Latin in Late Antiquity because to me it seems that the educated elite in the West studied Cicero no less to pick up good Latin (as opposed to the language spoken in the streets) than their Greek counterparts did with Demosthenes or Isocrates in order to pick up Attic Greek (in contrast to the Koine). Recently I happened across dual forms in Themistios - is what a Symmachus is doing really all that different or any less artificial?

And even in the first century we have Quintilian who clearly shows that there are differences in the language used in domestic contexts (otherwise it wouldn't be so important to choose a nurse with good Latinity; the elites strive to use a more educated/literary/elevated register even at home) and who testifies to the existence of a canon of "classical" Latin language models.