I don't know if I have the time, but I'll give it a shot.
Responding to [@HeySeuss] query.

I appreciate HS's question about the regions and your response to move around Vietnam. The only units that would do that are Super Secret Special Operations units, possibly working directly for MAC-V. Or are we going to do a series of vignettes with different characters each time assigned to different units? I guess what I'm trying to say is, if we are all members of some LRRP team in the 173rd Airborne Brigade (as an example) assigned to Bien Hoa Air Base near Saigon. This Brigade would not do operations in other Areas of Operations (AO). The Brigade was a Corps level asset which allowed them to operate in a larger area than a division, but still, they were restricted as to where they would go.

If you are suggesting we write all new characters assigned to different units every time we jump to a different region, sure, that's sounds cool. MAC-V (Military Assistance Command-Vietnam), is the theater command echelon containing the four-star general (William Westmoreland, initialy and later Creighton Abrams) commanding all US Armed forces in Vietnam. They have a unit that is not an entire corps of 50,000+ Joes that could go just about anywhere in Vietnam. That unit is the 5th Special Forces Group. They operate in 8-12 man teams lead by a Captain (đại úy - pronounced Die-Wee), 1st Lieutenant (XO) and a Master Sergeant (NCOIC) and spend most of their time working with indigenous populations (i.e.: the [url=https://en.wikipedia.org/wiki/Degar]Montagnards[/url]) training them how to fight against the NVA and VC. They occasionally were tasked with direct action missions like assassinations or raids.