This would be easier if I could put pictures up but i hope what i describe suits what you need. This is as simple a base as I can think of.
You want to do a "large" diorama, vietnam era? Awesome.
Get a piece of timber board the size of the diorama - often hardware stores will cut to size if they have the tools. I choose "MDF" wood - it is inexpensive, has no wood grain (so it wont warp or twist as it dries or gets moisture) and is easy to work with. Buy as thick as you can afford - 1/2 inch thick board thats 3 foot x 2 foot is about $10 (AUstralian) and most large dios would fit in that sort of shape. Unless its really large..
Anyways, Use an aerosol can of an approriate colour such as brown, black or olive drab to paint the outside of the board and some of the top around the edges. Let this dry
Then mask off the painted edge say an inch thick. the masking tape protects the painted surface underneath. Mask off the outer edge at the same time just to protect that aswell.
Now the exposed wood in the middle of the board is where you build the diorama. plaster and glue anything else gets stuck down there. (mix white PVA glue into your plaster landscapes). paint and detail the groundwork.
When the groundwork is finished/ landscaped, peel off the masking tape to reveal this nice clean ready made boarder. get the local locksmith to engrave a little brass tag (like a dog collar blank) with the title and glue it onto the frame.
Thats the most simple easiest basic base/ frame I can think of. I hope you understand what I mean as I simple dont have time to draw it up. But I hope gives you some ideas.
I avoid chipboard and pineboard as they swell when wet and can crack the plaster landscapes, or the pine warps and the plaster bases snap off. Assuming you use plaster of course to model the landscape..
Hope that helps
Adam