Vulgar Latin isn't filled with profanities or a slang version of Classical Latin—although there certainly were vulgar words. Rather, Vulgar Latin is the father of the Romance languages; Classical Latin, the Latin we study, is their grandfather.
Vulgar Latin was spoken differently in different countries, where, over time, it became such familiar modern languages as Spanish, Italian, French, Catalan, Romanian, and Portuguese. There are others less commonly spoken.
The Spread of Latin
When the Roman Empire expanded, the language and customs of the Romans spread to peoples who already had their own languages and cultures. The growing Empire required soldiers to be positioned at all the outposts. These soldiers came from all over the Empire and spoke Latin diluted by their native tongues.
The Latin Spoken in Rome
In Rome itself, the common people did not speak the stilted Latin that we know of as Classical Latin, the literary language of the first century B.C. Not even the aristocrats, like Cicero, spoke the literary language, although they wrote it. We can say this because, in some of Cicero's personal correspondence, his Latin was less than the polished form we think of as typically Ciceronian.
Classical Latin was, therefore, not the lingua franca of the Roman Empire, even if Latin, in one form or another was.
Vulgar Latin and Classical Latin
Throughout the Empire, Latin was spoken in many forms, but it was basically the version of Latin called Vulgar Latin, the fast-changing Latin of the common people (the word vulgar comes from the Latin word for the common people, like the Greek hoi polloi 'the many'). Vulgar Latin was a simpler form of literary Latin.
- It dropped terminal letters and syllables (or they metathesized).
- It decreased the use of inflections since prepositions (ad (> à) and de) came to serve in place of case endings on nouns.
- Colorful or slang (what we think of as 'vulgar') terms replaced traditional ones—testa meaning 'jar' replaced caput for 'head'.
You may see some of what had happened to Latin by the 3rd or 4th century A.D. when a list of 227 fascinating "corrections" (basically, Vulgar Latin, wrong; Classical Latin, right) was compiled by Probus.
Between the changes in the language wrought by the native speakers of Latin, the changes, and the interaction between Latin and the local languages, Latin was doomed—at least in common speech.
For professional and religious matters, Latin based on the literary Classical model continued, but only the well-educated could speak or write it. The everyday person spoke the everyday language, which, with the passing years, diverged more and more from even Vulgar Latin, so that, by the end of the sixth century, people from different sections of the Empire could no longer understand people in others: Latin had been replaced by the Romance languages.
Although both Vulgar and Classical Latin have largely been replaced by the Romance languages, there are still people who speak Latin. In the Roman Catholic Church, ecclesiastical Latin never entirely died out and has seen an increase in recent years. Some organizations deliberately use Latin so people can live or work in a living Latin environment. There has been a radio news broadcast from Finland that is delivered all in Latin. There are also children's books that have been translated into Latin. There are also people who turn to Latin for new names for new objects, but this only requires an understanding of individual words and is not a "living" use of the Latin language.
A Nosferatic Language?
There is no rule against academics taking their inspirations from B-movies, but this may surprise you.
Someone on the Classics-L email list referred to Latin as a Nosferatic Language. If you try Googling the term, Google will suggest Nostratic language, because Nosferatic is something of a punning neologism. A Nostratic language is a proposed macro-family of languages. A Nosferatic language is an undead language, like the vampire Nosferatu for whom it is named.
English and Latin
English has lots of words of Latin origin. Some of these words are changed to make them more like other English words—mostly by changing the ending (e.g., 'office' from the Latin officium), but other Latin words are kept intact in English. Of these words, there are some that remain unfamiliar and are generally italicized to show that they are foreign, but there are others that are used with nothing to set them apart as imported from Latin. You may not even be aware that they are from Latin.
Whether you want to translate a short English phrase (like "Happy Birthday") into Latin or a Latin phrase into English, you can not just plug the words into a dictionary and expect an accurate result. You can't with most modern languages, but the lack of a one-to-one correspondence is even greater for Latin and English.
Latin Religious Words in English
If you want to say that the prospects are bleak, you could say "it doesn't augur well." Augur is used as a verb in this English sentence, with no particular religious connotation. In ancient Rome, an augur was a religious figure who observed natural phenomena, like the presence and location to left or right of birds, to determine whether the prospects were good or bad for a proposed venture.