The Black Dog of Doom offers a set of free wargames rules for Vietnam Skirmishes.
The author writes:
This is a set of wargame rules for playing a middling size skirmish set in the American/Vietnam war. The rules were written for use with 20mm metal minis and have been used in that role a number of times.
As well as the figures and terrain, you will also need to supply some standard dice, a measuring stick, lots of tiddley winks and some crunched up balls of paper!
The rules feature the use of supressing fire which will pin all but the bravest of warriors. In game terms, supressing fire reduces the ability of the supressed to interfere with your plans.
Also included are some rules for handling vehicles and artillery fire, although these features are not expected be the focus of the game. In play each player will easily be able to handle 20-30 figures.