During the temporary absence or temporary disability of the City Manager, he or she shall, by letter filed with the City Secretary, appoint a qualified administrative officer of the City to perform his or her duties during such absence or disability. In the event of the incapacity or failure of the City Manager to make such appointment, it may be made by the City Council. During any vacancy in the office of City Manager, the City Council shall designate, by letter filed with the City Secretary, a qualified administrative officer of the City to serve as acting City Manager during the existence of such vacancy.