When you mean diorama, are you talking about just a static display of troops that will not see use?..or are you going to game with them using a set of rules?..
I think that the first thing you have to do is to figure out which scale minis that you wish to use. This is usually done by comparing availibility vs time you have vs how much you can spend.
The next part you would want to look at is, if you are going to be painting them, and how secure you are in painting. If you consider yourself a good painter and love to do small detail work, then you might want to look at 25mm, but they will be fewer and more expensive. I use 20mm myself, and the Rafm "Charlie Company line", and the Britannia Lines seems to be very good quality. Britannia seems to be increasing their Viet Minis lines almost monthly while Rafm, has had some distro and production problems recently, they are still trying to get back on track, yet they are still a good buy for basic figures.
Peter Pig also makes some good 15mm figs..and if you just want some soldeirs to play with and not spend lots of money, you can always go with the old Airfix/Esci 1/72 Plastic Vietnam guys.
Some other sites you might want to visit are the egroups sites for Vietgaming, and Charlie-Company. These are two sites devoted to Vietnam wargaming and not only discusses rules and figures but all sorts of material to build bunkers, jungle, terrain, and house rules, and official clarifications. See ya there trooper!
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Just wanted to know if anyone had any good advise on what brand of miniatures are the best for war gaming? I'm looking for good detail and accuratciy, I'm making a Firebase diorama.. Any help would be appreciated. Thanks Jeff..