We have literature from a bunch of these languages. Another user has already mentioned Langue d'Oc. We have tons of literature from Romance language not widely spoken today.

But before getting to that, a couple of things about your question need to be "corrected" for lack of a better term.

First, French, Spanish, et. al. are not "Latin Vernaculars" per se. They are languages that evolved from spoken or "Vulgar" Latin.

Second, and this is minor, French is not the vernacular of the Franks. They spoke a Germanic language, as did the Vandals, but both groups ruled over populations that spoke some form of Vulgar Latin/Later Latin. The Franks adapted to the linguistic traditions of those over whom they ruled. Latin Alive! is a good history of this. The Vulgate of St. Jerome is related to this idea of Vulgar or spoken Latin. It is written in a simple style meant to be accessible to Latin readers in the Western Empire. We have some very interesting examples of spoken Latin in the very Late Roman Empire. Egeria, a nun from the Latin speaking west wrote an account of her pilgrimage to the Holy Land in about 385 called the Itinerarium.

Taken from the supplementary material from Harrington's textbook, Medieval Latin :

"Most importantly, perhaps, the Itinerarium speaks to the ways in which post-Imperial Latin threw to the side the limitations of elite diction, genre, and syntax imposed from on high during the imperial period. MOre specifically, the coexistence of Vernacular Latin and Early Latin [the creators of this textbook draw a line between Vernacular Latin, spoken by Romans during the high imperial period, and "later Latin," a problematic term for Medieval Latin] in Egeria's work renders the Latin of the Itinerarium representative of the living language of the fourth century, a language mostly spoken and constantly in flux.

That flux is important to the development of LAter Latin in general, for it includes features common to the older version of the language but configured in new ways, including more freedom in the application of Classical Latin syntax and orthography and, in some instances, the appropriation of entirely new rules. Egeria is a witness to these trends in several ways. For specific cases: she writes per giro, per girum, per valle illa, and, for the place to which construction, in eo loco. Nor are these constructions simply the result of ignorance, for Egeria knows the uses of Classical Latin idioms. Instead, her syntactic changes often reflect the simplifying influence of Early Latin, which Egeria knew through Latin translation of the Bible."

And I'll stop there, because the authors get into some more serious grammar talk—this is a Latin textbook, after all.

As the Roman Empire disintegrated, the various dialects turned into related languages with their own rules. "Italian," for example, is descended from the Tuscan dialect, which heavily influenced the Roman dialect, particularly in the 17th century. But Sicilian, Ligurian, Venetian, etc. were all dialects that persisted into the twentieth century, and the speakers of these languages produced texts that can be read today. For example, Lu rebellamentu de Sichilia is a Sicilian language account of the War of Sicilian Vespers: https://books.google.com/books?id=KFEIAAAAQAAJ. As a further example, the anonymous Genoese poet left behind a collection of poems in the Ligurian dialect.

Many of these languages persisted into the 20th century and to this day. If you have read any of Elena Ferrante's tetrology, you know that the narrator talks about switching between "dialect" and Italian. She is referring to the local Neapolitan language. Fabrizio de Andre even recorded an album in Ligurian, or more specifically, Zeneize: https://open.spotify.com/album/6UGZ5FUUu2W0MWtEUvEcIJ?si=HiMcnYqGTTelMd9EvAMTqw. Although it's getting rarer and rarer, there are still native speakers of Ligurian in the mountain communities.

To conclude, at least Dante was aware of the mutability of Languages. In De Vulgari Eloquentia he talks about how not only are languages in Italy, which all came from Latin, different, but that people living several hundred years before him would be unable to understand speakers of those same languages today.