Vice admiral (United States)
In the United States Navy, the United States Coast Guard, the United States Public Health Service Commissioned Corps, the National Oceanic and Atmospheric Administration Commissioned Corps, and the United States Maritime Service, vice admiral (VADM) is a three-star flag officer, with the pay grade of O-9. Vice admiral ranks above rear admiral and below admiral. Vice admiral is equivalent to the rank of lieutenant general in the other uniformed services.
Statutory limits 
U.S. Code of law explicitly limits the total number of vice admirals that may be on active duty at any given time. The total number of active-duty flag officers is capped at 160 for the Navy. For the Navy, no more than 16.7% of the service's active-duty flag officers may have more than two stars. Some of these slots are reserved by statute. For example the Surgeon General of the United States Navy  is a vice admiral. The Judge Advocate General of the Navy  is a vice admiral in the Navy or a lieutenant general in the Marine Corps; the Surgeon General of the United States  is also vice admiral in the Public Health Service Commissioned Corps. Officers serving in certain intelligence positions are not counted against either limit, including the Deputy Director of the Central Intelligence Agency. The President may also add vice admirals to the Navy if they are offset by removing an equivalent number of three-star officers from other services. Finally, all statutory limits may be waived at the President's discretion during time of war or national emergency.
Appointment and tour length 
The three-star grade goes hand-in-hand with the position of office it is linked to, so the rank is temporary. Officers may only achieve three-star grade if they are appointed to positions that require the officer to hold such a rank. Their rank expires with the expiration of their term of office, which is usually set by statute. Vice admirals are nominated for appointment by the President from any eligible officers holding the rank of rear admiral (lower half) or above, who also meet the requirements for the position, under the advice and/or suggestion of their respective department secretary, service secretary, and if applicable, the Joint Chiefs. The nominee must be confirmed via majority vote by the Senate before the appointee can take office and thus assume the rank. The standard tour length for most vice admiral positions is three years but some are set four or more years by statute.
Extensions of the standard tour length can be approved, within statutory limits, by their respective service secretaries, the Secretary of Defense, the President, and/or Congress but these are rare, as they block other officers from being promoted. Some statutory limits under the U.S. Code can be waived in times of national emergency or war. Three-star ranks may also be given by act of Congress but this is extremely rare.
Other than voluntary retirement, statute sets a number of mandates for retirement. Three-star officers must retire after 38 years of service unless appointed for promotion or reappointed to grade to serve longer. Otherwise all flag officers must retire the month after their 64th birthday. The Secretary of Defense, however, can defer a three-star officer's retirement until the officer's 66th birthday and the President can defer it until the officer's 68th birthday.
Flag officers typically retire well in advance of the statutory age and service limits, so as not to impede the upward career mobility of their juniors. Since there are a finite number of three-star slots available to each service, typically one officer must leave office before another can be promoted. Maintaining a three-star rank is a game of musical chairs; once an officer vacates a position bearing that rank, he or she has no more than 60 days to be appointed or reappointed to a job of equal or higher importance before he or she must involuntarily retire. Historically, officers leaving three-star positions were allowed to revert to their permanent two-star ranks to mark time in lesser jobs until statutory retirement, but now such officers are expected to retire immediately to avoid obstructing the promotion flow.
United States Maritime Service 
Although it exists only as an auxiliary, the United States Maritime Service does maintain the rank of vice admiral. No USMS officer is currently commissioned at this rank. The last USMS officer to hold the grade was Joseph D. Stewart, who retired in 2008. Even though the Maritime Service is an auxiliary service, the grade is appointed by the President via the Secretary of Transportation, with the consent of the Senate, making it a federally recognized rank with corresponding paygrade. However, their pay differs from the other federal uniformed service pay scale.
- 10 USC 526. Authorized strength: general and flag officers on active duty
-  10 USC 525. Distribution of commissioned officers on active duty in general officer and flag officer grades.
-  Pub.L. 110-181: National Defense Authorization Act for Fiscal Year 2008
-  Pub.L. 110-181: National Defense Authorization Act for Fiscal Year 2008 full text
-  10 USC 5137. Bureau of Medicine and Surgery: Chief; Deputy Chief.
-  42 USC 207. Grades, ranks, and titles of commissioned corps.
-  10 USC 528. Officers serving in certain intelligence positions: military status; exclusion from distribution and strength limitations; pay and allowances
-  10 USC 527. Authority to suspend sections 523, 525, and 526.
-  10 USC 601. Positions of importance and responsibility: generals and lieutenant generals; admirals and vice admirals.
-  10 USC 636. Retirement for years of service: regular officers in grades above brigadier general and rear admiral (lower half).
-  10 USC 1253. Age 64: regular commissioned officers in general and flag officer grades; exception.
-  DoD News Briefing on Thursday, June 6, 1996. Retirement of Admiral Leighton W. Smith Jr.
|United States commissioned officer and officer candidate ranks|
|Pay grade / Branch of service||Officer
|Approximate insignia||(no universal insignia)|
|Air Force||Cadet / OT||2d Lt||1st Lt||Capt||Maj||Lt Col||Col||Brig Gen||Maj Gen||Lt Gen||Gen||GAF|||
|Army||CDT / OC||2LT||1LT||CPT||MAJ||LTC||COL||BG||MG||LTG||GEN||GA||GAS|
|Marine Corps||Midn / Cand||2ndLt||1stLt||Capt||Maj||LtCol||Col||BGen||MajGen||LtGen||Gen|||||
|Navy||MIDN / OC||ENS||LTJG||LT||LCDR||CDR||CAPT||RDML||RADM||VADM||ADM||FADM||AN|
|Coast Guard||CDT / OC||ENS||LTJG||LT||LCDR||CDR||CAPT||RDML||RADM||VADM||ADM|||||
|Public Health Service||||ENS||LTJG||LT||LCDR||CDR||CAPT||RADM||RADM||VADM||ADM|||||
|National Oceanic and
 Grade is authorized by the U.S. Code for use but has not been created
 Grade has never been created or authorized
|United States warrant officer ranks|
|Public Health Service|||||||||||
|National Oceanic and
 Grade is authorized for use by U.S. Code but has not been created
 Grade never created or authorized