Sounds good. One tip, Vietnam uniforms were not OD Green. They were a lighter green called OG 107 green. Testors Model Master Medium Green represents it well.
OG 107 Vietnam Jungle Fatigues. The pilot on the far left has on an OD Green flak vest.
Here are a couple Bravo 6 MPs in Medium Green to see how it looks. The M151A1 and vest on the driver are OD Green.
No issue with the 1/35 figures and 1/32 helo. They mix together well since the two scales are close. I mix them all the time with helos, vehicles, and figures. As long as you keep constant scale items (weapons, grenades, canteens, ammo pouches, etc.) in the same scale, there will be no issues. You can even mix 1/32 and 1/35 figures together since people come in various sizes as well.
Also, please don't use the long-haired hippie from the Verlinden set. He is pure Hollywood. Soldiers still had grooming standards to follow in Vietnam and no NCO would allow one of his soldiers to look like that.
Lastly, I think your base is way too big. I would go with one that is just a little bit larger than the helicopter. That way it is not dwarfed by the base and the focus is the helo, not everything going on around it. Maybe limit your figures to just a few helo crewmen too. The figures you have selected are more infantry figures. I think it would look better with a helo crew.
You could use CMK's Vietnam Helicopter Pilots set and have one leaning against the helo and the other calling out to him from the side of the base.
And/or the seated gunner from the Dragon Vietnam Helo Crew set (top of pic holding the free M60) sitting on one side adjusting the gun or soemthing, but not in a firing position.
Some ideas.