late 14c., "common, ordinary," from Latin vulgaris, volgaris "of or pertaining to the common people, common, vulgar, low, mean," from vulgus, volgus "the common people, multitude, crowd, throng," for which de Vaan offers no further etymology.
The meaning "coarse, low, ill-bred" is recorded by 1640s, probably from earlier use (with reference to people) in the meaning "belonging to the ordinary class" (1520s). Chaucer uses peplish for "vulgar, common, plebeian" (late 14c.). Related: Vulgarly.
What we have added to human depravity is again a thoroughly Roman quality, perhaps even a Roman invention: vulgarity. That word means the mind of the herd, and specifically the herd in the city, the gutter, and the tavern. [Guy Davenport, "Wheel Ruts"]
For Vulgar Latin, see here.
1940, a French word brought into English by John Buchan (Baron Tweedsmuir) and picked up by philosopher C.E.M. Joad because they found no adequate word in English for one "who spreads with clarity, vividness, force and accuracy, the knowledge obtained by and the wisdom derived from others" [Joad, 1948], vulgarize already being in use in the pejorative sense; see vulgar.
It has been pre-eminently the age of the vulgarisateur in the best sense of that word. I think the tendency wholly admirable. Lord Rutherford used to say that no conclusion which he ever reached was of any use to him until he could put it into plain English, into language understood by the ordinary man. Attempts to present the history of the world as an interrelated intelligible process, or to give a bird's-eye view of the long march of the sciences, may be faulty in detail, with many arbitrary judgments, but they do furnish principles of interpretation which enable the reader to find at any rate one way in the world of thought—perhaps a little later to make his own way. In this task the vulgarisateur may be preparing the soil for a rich future harvest, just as the work of the Sophists cleared the ground for Plato. [John Buchan, "Memory Hold-the-Door," 1940]
1570s, "the common people," from French vulgarité and directly from Late Latin vulgaritas "the multitude," from vulgaris (see vulgar). Meaning "coarseness, crudeness" is recorded from 1774.