Apparently, the Greeks of early Neapolis simply called it Somma, as in "summit"—in other words, "the mountain." It is not uncommon in many languages to call the most important something The Something. Other mountains have names—Harry, Bob—but this one is simply THE MOUNTAIN. It's archetypal and ominous. You can hear it rumbling. Even modern Neapolitan dialect may refer to Vesuvius as la muntagna. Indeed, the complete name of the most famous volcano in the world is the Somma-Vesuvius volcanic complex. It is a composite, really, made up of an older volcano, Monte Somma, the activity of which ended with the great eruption of 79 AD and a summit caldera collapse, producing the more recent cone, Vesuvius, contained within the caldera. Thus, before the great eruption, it was Mt. Somma; yet, there are ample Roman references to "Vesuvius" from before the great eruption. So, did they use both names? Or was there a gradual change from one to the other? Probably both.

One sources I am reading is Diario del Monte Vesuvio by Giovanni P. Ricciardi. He simply says, "Mt. Somma, Vesuvius to the Romans...." So, it seems straightforward. The Greeks called it "mountain" or "summit" and the Romans came along and called it Vesuvius, so we just have to figure out what Vesuvius means. But wait. A few paragraphs later, Ricciardi cites a fascinating passage from Berossus, the Babylonian historian, a rough contemporary of Alexander the Great. Berossus claims that one of the ancient kings of Assyria (in the year 1894 BC [sic!]) used "Vesuvius" in a reference to the fiery zones of Italy.

Ricciardi is a nuclear physicist with a passion for volcanoes, earthquakes, and, no doubt, accuracy. His quotes from Latin references to Vesuvius before the great eruption are fascinating: there are a number of references to Vesuvius as the site of the battle against Spartacus, and there are even a few comments on the morphology of the area to the effect that this thing looks as if it must have been a real volcano at one time or another! But Vesuvius must have been quiet for some time since there are no Latin or earlier Greek references to an eruption. Ironically, even Pliny the Elder, who listed active volcanoes in his Natural History, didn't mention the volcano that ultimately killed him in 79 AD.

I wonder why Ricciardi drops in the Berossus citation at the beginning of an otherwise legitimate list. As a scientist, he surely knows that literary types simply make stuff up. Thus it is with the Beroso Antiquity citation. It's a fake, written in 1498 by Annius of Viterbo, a Dominican friar and one of the great literary con-men of the Italian Renaissance. There may be some thrice-removed fragments from Beroso in existence, but they are all about the history of Babylonia. If he knew that the Greeks living in Neapolis in 300 BC called the volcano something other than the mountain in 300 BC, whatever he may have written about that has not survived. And any reference to what it was called by Italic tribes in 2000 BC is totally spurious. Not that it's not a good story.

Maybe the simplest answer is the most plausible. Roman writers used Vesuvius as well as Vesevius, Vesvius, and Vesbius. There happen to be Indo-European linguistic roots such as wes- and ves- that have to do with fire, hearth, light, etc. (As in the vestal virgins, guardians of the sacred flame in the temple of Hestia, goddess of the hearth.) So, mountain of fire, perhaps, driven by a religious need to move beyond the "Ugh-MOUNTAIN!" caveman stage of toponomastics to something a bit more imperial-sounding. There are other plausible derivations from similar roots that suggest "unquenchable" or "hurling violence." But deriving roots from languages that no longer exist is tricky business, and I am happy with ves- and "mountain of fire" as an early Roman coinage, although it does seem a bit too violent for a mountain that had not erupted in hundreds of years. Maybe it was more on the order of "nice peaceful hearth and that's just the way we like it."

There are certainly other, more fanciful etymologies:

—from a Roman mispronunciation of the Greek for "Son of Zeus," referring to Hercules, closely connected to Greek mythology in the Bay of Naples;
—from the Latin expression Vae suis! (roughly: Man, are we in trouble now!). That explanation was current around the time of the great eruption of 1631. It, too, is a good story, like the one with the Assyrian king, but on a scale of one to ten? Minus 6.