The board shall meet at the place to be designated by the board as often as the needs of the district require, on notice to each member of the board. Two members of the board shall constitute a quorum at any meeting. Any vacancy on the board shall be filled by the remaining member or members of the board, the appointee or appointees to act until the next biennial election at which directors are elected as provided in section 235.140, when the vacancy or vacancies shall be filled by election. If the board shall fail, neglect or refuse to fill any vacancy within thirty days after the same occurs, the county commission shall fill such vacancy.