The Battle of Monmouth Courthouse (June 28, 1778), was the last engagement for the 8th Virginia Regiment in the war. Shortly after, the seriously depleted regiment was folded into the 4th Virginia Regiment and Colonel Abraham Bowman was released as a "supernumerary" officer. (In fact, at Monmouth, the regiment had already been provisionally merged with two others as the "4th, 8th, and 12th Regiment.") Monmouth was fought exactly two years after the regiment's first battle at Sullivan's Island, South Carolina.

Very few of the original enlistees were still in the regiment at Monmouth. Aside from deaths from disease and battle, all of the original enlistments from 1776 expired during the Valley Forge Encampment. Some of the original officers still remained, however, including Colonel Bowman. Some of the original recruits had also reenlisted.  Monmouth, however, was the end of the road for the storied 8th Virginia, a unit that first began as a Virginia provincial regiment led by a pastor and still loyal (technically, at least) to the King. The Virginia legislature had intended it to be a German unit and commissioned German field officers for it (Col. Muhlenberg, Lt. Col. Bowman, and Maj. Helphinstine). It recruited men of other ethnicities, however, and was never as German as originally envisioned.

Some of the men, commissioned and enlisted, continued to fight on to the end of the war. The 8th was provisionally merged with the 4th and the 12th regiments during the summer of 1778 and fought at Monmouth with that designation. In September, the regiment was merged with the 4th Virginia under the latter's number. Other regiments were consolidated as well, and the regiment previously known as the 13th became the "new" 8th Virginia. In 1779, the consolidated 4th was provisionally merged with the 3rd Virginia and known for a time as the "3rd and 4th Virginia Regiment." Lastly, the handful who remained were included in the 2nd Virginia Brigade sent to reinforce General Benjamin Lincoln at Charleston, South Carolina, in 1780. Some of them were under the command of Captain Abraham Kirkpatrick, who had begun the war as a lieutenant in William Croghan's Pittsburgh company of the 8th Virginia. Croghan, now a major, was also at Charleston. All of them were taken prisoner when Lincoln surrendered on May 12, 1780. 

Brigadier General Peter Muhlenberg, the regiment's original colonel, and Lt. Colonel William Darke, one of the regiment's original captains, were both at Yorktown. They may be the only men of the original 8th Virginia who served at Yorktown as members of the Continental Army. Darke, who had been captured at Germantown, paroled and then exchanged, held a Continental commission but led militia from Hampshire and Berkeley counties to Yorktown. Malaria, to which men from the frontier had no resistance, prompted his militiamen to return home before the surrender of Cornwallis, but he remained (presumably having developed a resistance after the regiment's ordeal in Georgia in 1776). Private Bean Smallwood, an original 8th Virginia recruit in Captain Berry's company, was at Yorktown as a militiaman. 

Here is an excellent overview of the Battle of Monmouth. 
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A dozen different generals commanded the 8th Virginia in various capacities during its roughly 30-month existence.  The Continental Army grades of general officers were: general (Washington), major general (division commanders, typically), and brigadier generals (brigade commanders). The army was organized into departments: Canadian, northern, Highlands, eastern, main, and southern. Washington was the de facto commander of the middle (or "main") department for most of the war. Major General Charles Lee (junior only to Washington in the entire army) was commander of the Southern Department during the 8th Virginia's service in that theater.

A large number of 8th Virginia men were detached to the 1st Virginia for the entire 1776 campaign, under the command of Pittsburgh’s Captain William Croghan. While the rest of the regiment went south from Virginia to serve in South Carolina and Georgia under Lee, Croghan’s detachment went north to serve in New York, New Jersey, and Pennsylvania with Washington. In 1777 a select group of riflemen from the 8th were detached to Daniel Morgan’s Rifle Battalion under the command of Captain James Knox and participated in the Saratoga campaign. The main body of the regiment fought with Washington at Brandywine and Germantown. With its ranks severely depleted by disease, casualties, and expired enlistments, the 8th was folded into the 4th Virginia after the Battle of Monmouth.
 
1776 Campaign (Sullivan’s Island, Savannah, Sunbury):
 
General George Washington, Commander in Chief
Major General Charles Lee, Commander of the Southern District
Brigadier General Andrew Lewis (Tidewater service)
Brigadier General Robert Howe (Charleston, Savannah, Sunbury)
 
Croghan Detachment (White Plains, Trenton, Assunpink Creek, Princeton):
 
Major General Joseph Spencer (White Plains)
Major General Nathanael Greene (Trenton and Princeton)
Colonel George Weedon (temporary brigade at Fort Washington)
Brigadier General William Alexander, Earl of Stirling (White Plains through Trenton)
Brigadier General Hugh Mercer (Princeton)
 
1777 Campaign (Brandywine, Germantown, Valley Forge)
 
General George Washington, Commander in Chief
Major General Benjamin Lincoln (spring)
Major General Adam Stephen (Brandywine, Germantown)
Major General Gilbert du Motier, Marquis de Lafayette (Valley Forge)
Brigadier General Charles Scott
 
Knox Detachment (Saratoga):
 
Major General Horatio Gates
Major General Benjamin Lincoln
 
1778 Campaign (Valley Forge, Monmouth):
 
General George Washington, Commander in Chief
Major General Gilbert du Motier, Marquis de Lafayette
Brigadier General Charles Scott