Vulgar didn’t always refer to making explicit/offensive references to sex/body functions, which I think is often forgotten. Vulgar used to refer to the common tongue of the masses, which is what this book is a dictionary of. The language of slums, and dockyards, and taverns, and the common folk just scraping by in England.
This was reading a dictionary, which was enjoyable in its own way. So many of these words we don’t hear anymore, and others I wish I had read before other books I had read this year (for example, learning “cove” in this dictionary before encountering it in Beka Cooper. Similar with "mort" meaning woman, which was transformed to "mot" in Beka Cooper - the better to avoid the correlation with the French "mort"="death"). Then there were also the words we do still use, and some of the most thematically common ones were grouped for modern-day readers all in a page or two in addition to being a dictionary entry.
I think this could have been improved on with some sort of historical forward, adding more contemporary context to the dictionary, instead of just pulling a few compilations of different slang which means the same thing (usually containing one or two popular words we still use today).