A complaint filed with the state's Fair Political Practices Commission says Vang violated regulations during the city's most recent budget process.
It states Vang wanted to fund a study that would potentially reduce traffic on a portion of a road near his home in south Fresno.
But regulations prohibit elected officials from taking part in decisions in which they could benefit.
Vang said his vote was for the city's budget as a whole and not that specific motion.
He went on to say the mayor is responsible for crafting the budget, and any motions made by councilmembers are merely suggestions.