If a vacancy occurs in the office of a city commissioner or president of the board of city commissioners, the board may call a special city election to fill such vacancy for the unexpired term, or may, after fifteen days from the date of such vacancy appoint a person to fill such vacancy until the next city election, at which election the unexpired term shall be filled. Upon petition of five percent of the qualified electors, as determined by the total number of votes cast in the city in the last city general election, the commission shall call a special city election to fill a vacancy occurring more than six months prior to the next city election, provided such petition has been submitted within fifteen days of the date of such vacancy.