There shall be elected at each annual meeting of the association, or at some special meeting thereof called for that purpose, 3 assessors, members of the association, to be known and called the board of assessors, who shall hold their office until the next annual meeting. In case of a vacancy in such board, caused by death, resignation, failure to accept office, or otherwise, the board of trustees may fill such vacancy by the appointment of some suitable member of the association, who shall hold the office until the next annual meeting.