At any time that a vacancy on the board of directors of a bank occurs, the bank shall, within thirty days, notify the department of the vacancy. Vacancies shall be filled within ninety days by appointment by the remaining directors, and any director so appointed shall serve until the next election of directors, except that if the vacancy created leaves a minimum of five directors, appointment shall be optional. When the vacancy has been filled, the bank shall make application to the department for approval of the director appointed in accordance with section 8-126.