Hey, I've ordered about 13 Ultimate Soldier 1:18 Vietnam figures and I have 7 right beside me. I've decided to try and make a fire base for them but have no idea where to start! I've found some small items in 1/16 scale but I'm trying to hunt down tents or hooches, some small shack-like buildings and sandbag bunkers. Does anyone know any companies that make 1:18 scenery or will I be forced to use 1/16 items? Thanks.

Give these guys a try Build-a-rama

Hooches and other buildings should be easy to scratch build. We essentially scratch built them in the 1:1 scale. I once spent a night in a building made of corrugated aluminum sheets of Hamm's beer can labels. It was 2x4 framed with the corrugated sheets tacked on to them.
If you do a little research I know there was an Army FM on building sandbagged emplacements. Artillery Field Manuals (FM) start with number 6. Firebase plans would be FM 6 - something or other. Some of these emplacements required thousands of sand bags. We also used 2x4s and 4x4s. PSP (pierced steel planking) was widely used as the base of overhead cover as covered in the FM.
Good luck in your project.