Vettonianus had an inauspicious start to his career, as a "tresviri capitales", one of the magistracies that comprised the "vigintiviri". This was the least desirable office of the four, for men who held that office rarely had a successful career: Anthony Birley could find only five "tresviri capitales" who went on to be governors of consular imperial provinces. Following this he was commissioned as a military tribune of the Legio VI Victrix, then stationed at the city of Legio (modern-day León) in Hispania Tarraconensis. After serving as a quaestor in Sicily, he accepted another commission as the "legatus legionis" or commander of the Legio IV Scythica in AD 62, stationed at Zeugma in Syria. At one point Vettonianus found himself commanding the legion under the leadership of Lucius Caesennius Paetus in the Roman–Parthian War of 58–63. The legion, however, was defeated in the Battle of Rhandeia and suffered dishonor.