Vietnam Landscaping

As far as the treeline goes (and the smoke for that matter)- you might just want to get some posterboard or if you want something a bit sturdier try masonite as it's a very smooth wood (clipboard kinda material) and then attach it to your base (constructed as stated ^) and then either freehand a landscape or depending on your artistic skills you could always get a good sized picture and glue it down to that. Get some purple/yellow/red paint, whichever you like or whichever makes the most sense with SAS hot extraction SOPs and paint on a little tuft of it flowing along the ground

As for rotar wash (with grass and smoke)- watch any classic vietnam movie- Apocalypse Now comes to mind, and you'll get the idea- if the smoke grenade is still producing smoke- it will be streaming out in an almost solid line away from the chopper, I'd have to guess though if the grenade was "done" there'd be no residual smoke left once the helo got close

I have not made many dioramas but I would agree with what the other chaps said...leave your troops aiming of the base. Re the smoke....would it have emptied by the time your helo touched down?? Maybe a coloured stain on the base with coloured grass around it?? Good luck with it mate

1) What is the best material to build a base from.

I use drop-ceiling tiles, the 2 x 4 foot types for big dioramas.

 2) How is the flattish but realistic terrain best portrayed (plaster?,styrofoam?)

I use blocks of wood or foam overlaid with plaster-soaked paper or cloth to form the "earth", then cover with celluclay... 

3) What is available in the way of longish reed like grass as seen in Vietnam

I would use the bristles from a cheap four-inch painbrush..

4) What is available in the Way of Jungle like trees or how do I build these realisticly

There is no quick answer for that.. Triple Canopy jungle is a beeatch and you'd be better off forgoing the trees in the PZ anyway... Helicopters don't like trees..

5) Any Ideas on replicating coloured smoke? I was thinking cotton wool spread out thin and    airbrushed lightly with the colour

Cotton would work, ya.. as long as it doesn't get too thick and look like cotton again.. Remember that the rotor wash is going to blow thesmoke away...

6) Any other Tips or Tricks on Diorama building or with this concept?

Like AJ said, buy Shep Paine's book..Has everything you need to know..

I would consider foregoing the NVA/VC/tree line. Just a base for the chopper and SAS is goinng to be huge. Giving adequate clearance for landing/take-off for the helo is going to expand your base immensely and leave a LOT of dead space. Plus the weigh of the thing is going to start coming into play.

That said, a piece of wood, either pre cut, like a plaque, or plywood will make a base. If plywood, you'll need quarter round to finish it off and it will need to be sealed to prevent warping.

I use Celluclay instant papier mache for my ground work. For your application you'll need a relatively smooth layer.

For grasses, there are plenty of products at Military Miniatures Warehouse but given the area you need to cover, dismantleing a broom might be better. Remember, the grass around the chopper is going to need to be flattened in a circular pattern due to rotor wash. If you go with denser vegetation, that site has some of that as well. Again, I'd reconsider including the treeline.

As far as smoke goes, yours is as good as any idea.

As far as other tips, sketch it out before hand. Use your digital camera to test out positions of your elements. Read up on artistic composition. Find a copy of Shep Paine's book on diorama building.