I heard a great voice. Bossuet explains the pouring out of the seven vials in this manner. The five first he supposes to have taken place under the reign of Valerian and Gallien; the sixth he supposes to have been poured out during the reigns of Valerian, Dioclesian, and Julian; and the seventh under Honorius and the Alani. (Bossuet)

All commentators, however, seem to agree that the great city mentioned in the 19th verse, is to be understood of Rome, and that the plagues which are here foretold, are denounced against her. (Calmet, Pastorini, &c.)

Go and pour out the seven vials, &c. According to the exposition followed by the bishop of Meaux[Bossuet], all these seven vials are already past, being punishments and judgments exercised against the heathen emperors, from the time of Valerian even to the time of Julian, at whose death it might be said, (ver. 17) it is done. Idolatry is destroyed, as to its public worship. Here in particular, by the drying up of the Euphrates, and by the armies of the East, these interpreters understand those of the Persians, who first gave the great shock to the empire in Valerian’s time, and by whom afterwards Julian the apostate was defeated, and killed. By the great Babylon they also understand idolatrous Rome; and by the islands and mountains sunk by earthquakes, they understand the destruction of divers kingdoms. According to another interpretation, (which is very common) all these judgments are to come before the end of the world; and will be in a manner literally executed about antichrist’s time. At the first vial, men shall be struck with ulcers and wounds, not unlike to the sixth plague of Egypt. At the second and third vial, the sea and fountains shall be turned into blood, as in Egypt. At the fourth vial shall be excessive scorching heats, tormenting men, and burning every thing for their use. At the fifth vial darkness, like that of Egypt. At the sixth vial, (ver. 12.) the Euphrates dried up, to open a passage for the armies from the East, to come and join the forces of antichrist. And the three unclean spirits like frogs, may signify devils sent by the dragon, or chief of the devils, to excite the wicked to all manner of unclean abominations. They are here said to be gathered together in a place called Armagedon, perhaps with an allusion to Mageddon, in the tribe of Manasses, where the two kings of Israel, Ochozias and Josias, perished. (4 Kings ix. 21.) And they are brought in only to signify a place of great destruction. See also Zacharias xii. 11. At the seventh vial, a voice, it is done, i.e. the reign of the wicked in general, and of antichrist, is at an end. (Witham)