A vacancy occurs in an office by reason of the death of the incumbent. The date of the death may be established by the date shown on the death certificate. A vacancy occurs in an office by permanent physical or mental disability rendering the person incapable of performing the duties of the office. The corporate authorities have the authority to make the determination whether an officer is incapable of performing the duties of the office because of a permanent physical or mental disability. A finding of mental disability shall not be made prior to the appointment by a court of a guardian ad litem for the officer or until a duly licensed doctor certifies, in writing, that the officer is mentally impaired to the extent that the officer is unable to effectively perform the duties of the office. If the corporate authorities find that an officer is incapable of performing the duties of the office due to permanent physical or mental disability, that person is removed from the office and the vacancy of the office occurs on the date of the determination.