In case of a vacancy in the membership of the board of finance, unless otherwise provided by charter or special act, the remaining members of the board shall, at a special meeting called by the chairman for that purpose, by a majority vote, appoint a successor, who shall serve until the next town election and until his successor is elected and has qualified. At the next town election following the occurrence of such vacancy, the electors shall elect a successor to fill the vacancy for the unexpired portion of the term and the office to be filled shall be designated on the official ballot by the words “To fill vacancy for ….”, followed by the duration of the unexpired term.