If ground-action is your game, and the plethora of helicopter dioramas rubs ya wrong (it would me, since "everyone is doing one"), keep in mind that, "combat is hours and hours of boredom, punctuated with a few minutes of sheer terror"...
That said, showing GIs living, eating, and sleeping in the field is always the best route, IMHO... Some expamples might be:
The Marine 105mm howitzer section doing mantenence on the gun, "punchin' the tube", the assistant gunner tearing down and cleaning the breech-block with the Number 1 Cannoneer, while the Section Chief (Redleg-speak for a Squad-leader) is telling the rest of the crew what ammo he wants broken out and readied...
The Army M-48 crew that's "busted track" to change an inside roadwheel or torsion-bar back at the firebase (wouldn't happen while they're on the hunt), while the lazy Spec-4 from the Company's Battalion Maintenence Section is sitting on a crate and while he watches (with the replacement parts), talks to the TC, ("Uh, uh-Nope... Bustin' track's a crew task, Sarge"...)...
The Army M113/M113 ACAV that's getting refueled... Fuel-dawgs are pumpin' gas either into the tank or into fuel-cans, as the driver does his fluid-levels, checks, and services and the TC loads water and/or C-rats into the back, etc...
The Army 5-ton or 2 1/2 ton Gun-truck that's getting ammo for the quad-fifites up-loaded for the next convoy escort while the truck's driver is getting a route-briefing and adding the checkpoints to his map. The new Platoon Leader ( a brand-new, Cherry "TwoEl-tee" ( who's having to drive his own 1/4-ton today because his driver took an AK-round in the forehead a couple days before, as evidenced by the bullet-hole through the windshield and dried blood on the seat-cushion) is going it over with him on the 1/4-ton's hood..
A squad from 3rd Marines, one that's laggered-up near the DMZ.. They're cleaning weapons, eating chow, shootin' the breeze, and snoozing (only one guy is sleeping, though.. The squad hasn't been outside the wire long enough to sleep in the heat yet) near a knocked-out Army M113 that the Army hasn't recovered yet, and which now boasts a field-expedient "lean-to" of snapped-together ponchos, a couple saplings, and some 550-chord or commo-wire, for shade in the murderous heat...
The Navy PBR-crew that's performing it's "Sun-Conditioning" on the deck.. Sun-Conditioning is a necessary part of life in the Brown-water Navy, and is performed in a reclining postion, bare-chested, with eyes closed, partially closed, or (most often), hidden behind sunglasses or boonie hats, while maintaining an "adult beverage" carefully balanced on the stomach.
(Pardon me now, if I get a bit maudlin... If ya just gotta do a helicopter)
The lone Crew-chief mopping the blood out of the back of his UH-1 "Dust Off" as it sits with the fuselage in a small creek, so that the blood will wash down-stream, and he doesn't leave a big puddle of red on the flight-line...
There're dozens and dozens of little things to model when you're depicting life in the bush, and while "Bang-bang-shoot-'em-ups" are visually "exciting", they don't really reflect what the overall "existance" is like for GIs in the field.. Your WW2 experience should still come in handy, since not much had changed between then and the Vietnam War... The tactics, weapons (well, most of the weapons), uniforms,equipment, aircraft ( again, most aircraft), and vehicles, did, sure... But the GIs were the same, be it 1944 or 1967, and so's the field.. The only real difference is the numbers and types of "creature-comforts" available...
This photo could have been taken in Vietnam in '67 or Sicily in '43... All that needs to change to Veitnam is the GI's weapons, uniforms, and battle-rattle, and change the vehicle to an M113 and ya got it... The helmets,canteen-cups and squad-stove haven't changed, not even the 45 holster ('cept it was black instead of brown by then)..
Look at some photos, and pay close attention to what is actually happening around the photo, rather than just look for uniform or equipment details..