The President may appoint, by and with the advice and consent of the Senate, one Vice Commandant who shall rank next after the Commandant, shall perform such duties as the Commandant may prescribe and shall act as Commandant during the absence or disability of the Commandant or in the event that there is a vacancy in the office of Commandant. The Vice Commandant shall be selected from the officers on the active duty promotion list serving above the grade of captain. The Commandant shall make recommendation for such appointment. The Vice Commandant shall, while so serving, have the grade of admiral with pay and allowances of that grade. The appointment and grade of a Vice Commandant shall be effective on the date the officer assumes that duty, and shall terminate on the date the officer is detached from that duty, except as provided in .section 306(d) of this title