His early professions are unknown, but it is evident that at some point he joined the military and must have greatly distinguished himself to rise through the ranks to the army's highest office, magister militum, by emperor Constans, despite being uneducated and unconnected.

In early 350, the commander of the famed Ioviani and Herculiani units, Magnus Magnentius, rose in rebellion and had Constans assassinated and himself crowned as emperor.

[4] She most likely thought Vetranio could protect her family and herself against the usurper, and merely hoped to secure his fidelity, though Edward Gibbon credits her notoriously unscrupulous ambition for the scheme, suggesting interested motives on her part.

Drinkwater still argues that ultimately Vetranio remained loyal to the Constantinian dynasty, and this idea is shared by Alan Dearn, who looked at the numismatic (coin) evidence, and Andrastos Omissi.

Such was the choice made in 361 by Lucillianus, the magister militum in Sirmium, potentially the exact same city in which Vetranio was raised to the purple.

[7] Understandings for the sequence of events, and the real motivations behind them, again hinge on whether one believes Vetranio was a Constantinian loyalist or a self-interested usurper.

Eventually, Constantius left from the eastern frontier with a large army, and met with ambassadors from both rebel emperors at Heraclea in Thrace.

They offered him the senior title in the Empire, and Magnentius proposed to wed his daughter to Constantius, himself to marry Constantina the emperor's sister.

Though dismissed from his command, he was allowed to live the remainder of his years as a private citizen on a state pension in Prusa ad Olympum, Bithynia.