how should i make sand for a small vietnam scene

Sandblasting sand come is some really fine grits.

There's a school of thought that says that something life-size in 1:1 scale cannot be lifesize in 1:35 scale.  You can pickup stuff at your pet store called "gecko sand" which, as the name suggests, is the stuff you put inside your tank with your pet reptile.  Its very fine grade, clean, and comes in a few colours.  I've used it before and I think it looks OK.

very fine sand will do the trick. And by the way, Dragon and Verlinden make some nice Vietnam figures. do a search on ebay or something like that and you'll probably find them.