Why do you think it WASNT dominated by Army units?

So the way US field forces were deployed was a mirror of the ARVN Corps structure. We assigned units to deploy to the same area as they had a Corps are of responsibility.

At various times divisions would be assigned to those commands, and the USMC presence was mostly concentrated in the Northern Highlands.

Various divisions rotated through at various times, but the height of the war in 1965-1972 involved part or all of the 1st Marine Division and 3rd Marine Division.

While for the Army, the 1st Infantry, 1st Cavalry, 4th Infantry, 9th Infantry, 25th Infantry, and the 82nd and 101st Airborne Divisions. Along with a handful of independent Brigades and Regiments.

The Army wasn't big enough, when you consider the other spots that we're keeping track of the Army's attention.

50-60,000 Army troops were stationed in Korea, keeping an eye on the lads on the other side of the border, with a number kept in reserve to go deal with any further DPRK naughtiness. About a quarter million troops were in Germany alone, not counting soldiers in any other NATO countries. Recall that the Berlin Wall went up only a few years before Vietnam kicked off, so the Army's primary focus was very definitely in that theater.

That said, I wonder if you're not understating the Army's role. By the end of 1967, with US military forces in Vietnam approaching a half million, seven of the Army's 18 divisions were in Vietnam, and Army personnel accounted for just under 2/3 of all US military forces.

There is one additional point, thoug I am unsure as to how much relevance this had at the decision-maker level. The fighting may have been non-amphibious, but it was generally low intensity stuff while the Army was better suited to the more traditional "kill people and breaks things" high intensity conflict. The Marines, however, had a history of what it called "small wars", and, indeed, were the only branch to have written a manual on counter insurgency at that point, drawing heavily on its experiences in the 1920s and 1930s.

Since WW2, Marines have had a dual doctrinal mission. First, have a special capability in expeditionary warfare. Second, utility tactical deployment largely interchangeable with light and medium Army units (in the case of Marine ground forces) and Navy aviation units (in the case of Marine Aviation).

Without the latter mission there wouldn't be a budgetary justification for anywhere near the size of the present USMC, and the Marines like to be big.

Interestingly the same is also true of the Airborne units of the Army. They train for helicopter and parachute deployment but have to be ready to rotate in for and interoperate with the conventional ground divisions and brigades.

This question seeks a more interesting answer than there actually is, since the Marine Corps presence in Vietnam was proportional to its size.

First, the premise of this question is based on an incorrect assumption that the Marines were the amphibious branch of the military. The Army actually conducted the largest amphibious invasions in our nation's history, including North Africa, Italy, and Normandy. The Marine Corps is more accurately described as America's expeditionary force. As their Marine Corps Hymn proclaims, "From the Halls of Montezuma To the shores of Tripoli; We fight our country's battles In the air, on land, and sea;"

OP would probably be surprised to learn around a third of Marine Corps officers were pilots. After World War II, the Army Air Corps became the Air Force. The Air Force did not inherit the Marine equivalent, and the Marines still have fighter jets and C-130 in addition to their helicopters. So while the Army had to work closely with the Air Force, the Marines could provide their own air support and air to air combat capabilities.

The first Marine Corps combat unit to arrive after the Gulf of Tonkin Incident was the 3rd Marine Expeditionary Brigade (MEB) which landed by air and sea in Danang. The MEB eventually grew to a Marine Expeditionary Force (MEF), which is one combat division, one logistics group, and an air wing. The MEF had responsibility for the northern quarter of South Vietnam known as I Corps, and later army units that were deployed there were put under MEF command. So even though the Marines had strategic command of about a fourth of the Country, not all ground units there were Marines.

You can check out this infographic to confirm the size of the Marines compared to the area they controlled was relatively proportional compared to the size of the Army:

http://historyinpieces.com/research/us-military-personnel-1954-2014

Anything to do with marines being volunteers? Or were they drafting as well?