In the event a vacancy occurs amongst participating commission members, the participating commission which designated the member whose seat has become vacant shall be entitled to fill the vacancy for the unexpired portion of the term, within 35 days from the occurrence of such vacancy. In the event said participating commission fails to fill the vacancy within the time period required, the Board of Selectmen shall be entitled to fill said vacancy for the unexpired portion of said term, and in the event said vacancy is filled by the Board of Selectmen, said member need not be a member of the participating commission.