Wikipedia
Vetranio (; died c. 356), sometimes incorrectly referred to as Vetriano, was a Roman soldier and statesman, a native of the province of Moesia (in modern Serbia). He was an experienced soldier and officer when he was asked by Constantina, the sister of Roman Emperor Constantius II, to proclaim himself Caesar (March 1, 350). Her brother Constans had been killed by Magnentius earlier that year and she probably thought Vetranio could protect her family and herself against the usurper. Vetranio accepted, and coins were minted in his name, showing the title of Augustus (full emperor), rather than Caesar, and the coins indicated that he expected to rule for five years, and hoped for ten. Constantius first seemed to accept the new Emperor and his regalia and sent him money to raise an army. Vetranio frequently asked Constantius for money and military aid to fight Magnentius; he also sent letters to the emperor professing his loyalty to him. When negotiations deadlocked, Magnentius and Vetranio made an alliance; they sent an embassy to the emperor. The ambassadors met with the emperor in Heraclea in Thrace. Magnentius offered to marry his daughter to Constantius and to marry Constantia the emperor's sister, in turn. But first, they required that the emperor lay down his arms and accept the honor as senior emperor. Constantius rejected their offer.
Constantius was on a campaign against the Persians when Magnentius came to power. Constantius first met with Vetranio at Serdica, and both moved on to Naissus in Serbia. On 25 December 350 both men mounted a platform before the assembled troops; Constantius managed, by means of a strong speech, to have the soldiers acclaim him emperor. He then took the purple away from Vetranio. The emperor led the old man down the stairs of the platform, called him father, and led him to the dinner table. He was allowed to live the remainder of his years as a private citizen on a state pension in Prusa ad Olympum, Bithynia.
See Philostorgius 3.22 for detail on his rise to power.