There's an Italeri UH-34D in 1/72. The kit could use some detailing, and it will probably need some putty. However, it's a good representation of a Marine Corps classic.
Fujimi makes a CH-46 which may be appropriate for Vietnam, but I'm not sure. I'm sure one of the Hueys on the market would fit the bill too.
I'm affraid there's no CH-53A's or D's around, but an Italeri HH-53C could be modified for USMC use. I don't think that would require very much surgery.
CH-37's aren't available in 1/72. I don't think anyone makes a vaccuum formed kit or resin. I know nobody makes it in plastic.
Semper Fi,
Chris