You might expect that something as catastrophic and traumatic as the eruption of Vesuvius would leave a huge mark in the historical record, but we really don't have all that much in terms of Roman accounts or views of it

We have one surviving eyewitness account of the eruption of Vesuvius, written by Pliny the Younger around twenty years later. Pliny's uncle (naturally enough known to scholarship as 'Pliny the Elder') died leading a rescue mission into the Bay of Naples, and was most famous as the author of the Natural History, a colossal encyclopaedia which aimed to collate all knowledge on (loosely) 'scientific' topics. In his account, the younger Pliny sets himself up as his uncle's intellectual heir-cum-acolyte, and makes a point of presenting himself as an objective, empirical observer of events - the reason we know Vesuvius-like eruptions as 'Plinian' is because he recorded the vulcanological signs accurately and in great detail.

Pliny's account was part of a letter to the great Roman historian Tacitus, whom Pliny claims had asked him to write something to allow him to write Pliny the Elder's part in the eruption into a history book. As far as we know, Tacitus never did, and the only other Roman historian to tackle the eruption in detail was Cassius Dio, writing in Greek in the early 3rd century AD. There are also passing references to the eruption in other historians, of which the most notable is the imperial biographer Suetonius, who wrote around AD 120.

The idea of a natural disaster being an omen would have been completely natural to an ancient Roman. Omens were an ordinary part of life, taken before any important event or decision, and we find references to them all over even the most hard-headed of Roman history writers. Tacitus, for example, is about as humanistic as Roman historians get, and is careful (in Histories 2.78) to record the omens that encouraged Vespasian to make his successful bid for the throne, even if he treats them as reports rather than literal truth and shows as much concern for their effects on Vespasian's followers as to their theological significance. Similarly, in his account, Boudicca's destruction of Colchester was foretold by a whole range of meteorological and other weirdness:

Meanwhile, for no apparent reason, the statue of Victory at Camulodunum fell, with its back turned as if in retreat from the enemy. Women, converted into maniacs by excitement, cried that destruction was at hand and that alien cries had been heard in the invaders' senate-house: the theatre had rung with shrieks, and in the estuary of the Thames had been seen a vision of the ruined colony. Again, that the Ocean had appeared blood-red and that the ebbing tide had left behind it what looked to be human corpses, were indications read by the Britons with hope and by the [Roman] veterans with corresponding alarm.

With that in mind, it's interesting that the eruption of Vesuvius wasn't generally seen as a bad sign by later writers. Here's Suetonius' mention of it in his biography of Titus:

There were some dreadful disasters during his reign, such as the eruption of Mount Vesuvius in Campania, a fire at Rome which continued three days and as many nights, and a plague the like of which had hardly ever been known before. In these many great calamities he showed not merely the concern of an emperor, but even a father's surpassing love, now offering consolation in edicts, and now lending aid so far as his means allowed. He chose commissioners by lot from among the ex-consuls for the relief of Campania; and the property of those who lost their lives by Vesuvius and had no heirs left alive he applied to the rebuilding of the buried cities. During the fire in Rome he made no remark except "I am ruined," and he set aside all the ornaments of his villas for the public buildings and temples, and put several men of the equestrian order in charge of the work, that everything might be done with the greater dispatch.

We need to think of Roman historians as literary artists first, and as everything they write as motivated by concerns that we might think of as literary or rhetorical as much as historical. In both of our sources above, it's clear that omens fill a role - for Tacitus, the omens before the destruction of Colchester help to increase the sense of suspense, terror and threat, while also perhaps suggesting the harsh justice of Boudicca's massacre of the abusive Roman colonists. In Suetonius' account of Titus, however, the disasters provide an opportunity for Titus to show his best qualities - generosity, compassion and efficiency in a crisis. Cassius Dio's account is similar - it goes into much more detail about the horror of the eruption, but doesn't suggest that it was a bad sign for Titus' reign - rather, he moves straight into praising Titus' response to it.

Both Suetonius and Dio were writing after Titus' death, and we might fairly suggest that the reason they don't jump on the eruption as a bad omen is because Titus's memory was overwhelmingly well-regarded, and both had a highly positive view of him. The very nature of omens means that they are often defined or reinterpreted in hindsight. Suetonius records that both Rome and Capua were struck by lightning on the Ides of March before Caligula was assassinated - and that this was taken as a sign of impending trouble, but we can just as well imagine it being interpreted as a good thing if something positive had happened afterwards, or indeed being forgotten about if nothing of interest occurred. It certainly wouldn't have suited their credibility as historians to call the eruption of Vesuvius a bad omen, given that the general memory of the rest of Titus' reign saw it as fortunate.

There are a few hints, however, that people at the time did see the eruption as some sort of apocalyptic event or as a sign of divine wrath. This, for instance, is from Dio's account:

Some thought that the Giants were rising again in revolt (for at this time also many of their forms could be discerned in the smoke and, moreover, a sound as of trumpets was heard), while others believed that the whole universe was being resolved into chaos or fire. ...

While this was going on, an inconceivable quantity of ashes was blown out, which covered both sea and land and filled all the air. ... Indeed, the amount of dust, taken all together, was so great that some of it reached Africa and Syria and Egypt, and it also reached Rome, filling the air overhead and darkening the sun. There, too, no little fear was occasioned, that lasted for several days, since the people did not know and could not imagine what had happened, but, like those close at hand, believed that the whole world was being turned upside down, that the sun was disappearing into the earth and that the earth was being lifted to the sky.

This would have been entirely understandable - eclipses were generally considered bad news, both in the Greek tradition and in the Roman one. Writing at the end of the 1st century BC, Livy (44.38) tells us of a scientifically-minded Roman tribune before the Battle of Pydna in 168 BC, who knew that an eclipse was coming and so took pains to teach his soldiers the astronomical explanation for it, 'so that none of them would take it as a bad omen.' The story itself may well be apocryphal, but it does suggest something about how ordinary Romans would generally think of the sky going suddenly dark.

To wrap all this up - as far as we can tell, the general reaction was clearly shock and horror, but most of our sources only really view the event in hindsight, which makes it difficult to judge what people thought would happen next. In the historical record, there's no sense that it was viewed as a bad omen or as a sign of divine displeasure, but this certainly wouldn't have been out of character for the Romans, and there are suggestions in the source material of contemporaries fearing that it was either a warning, or the beginning, of the end of the world.