I think by theatres, he means scenarios - e.g. The jungle conflicts, the vietnam villages, city wastelands, grassy mountains etc.
You'll need to find a good nam reference site to get all that info, then watch Appocolypse Now, Full Metal Jacket, We Were Soldiers, Hamburger Hill, Platoon etc. They're all based in different theatres and will give you a good contrast. We Were Soldiers is a nice clearing based combat, Platoon is very jungle, Full Metal Jacket gives the desolate city vibe, Appocolypse Now will help drop you into the insane drugged up random war type vibe. All good for inspiration.
Edit: This seems to be a decent site
http://www.pbs.org/battlefieldvietnam/