From Halopedia, the Halo wiki
Vice Admiral, abbreviated to VADM, is a senior commissioned officer rank of O-8 in the United Nations Space Command Naval Command (NAVCOM) rank structure; including the UNSC Navy and Office of Naval Intelligence. It is preceded by Rear Admiral, and subordinate to an Admiral.[1]
Command of Sector fleet or academy.[1]
Enlisted: Crewman Recruit • Crewman Apprentice • Crewman • Petty Officer Third Class • Petty Officer Second Class • Petty Officer First Class • Chief Petty Officer • Senior Chief Petty Officer • Master Chief Petty Officer
Enlisted: Private • Private First Class • Lance Corporal • Corporal • Sergeant • Staff Sergeant • Gunnery Sergeant • Master Sergeant • First Sergeant • Master Gunnery Sergeant • Sergeant Major