Enter me stage left, I agree completely with your statement the Chaffee wasn't used during the period of US involvment in the war, it was the bulldog but I am right now working on a mod to add more units, it's a difficult process due to me being new to modding this series of games but I have made some progress as I can now use the Phantom (will be difficult to use), those forests are dense, the UH-1 and Uh-1b (Transport version) and the Dodge M37 truck, here's a picture:
As you can see there is alot to do like putting them in the sections where they belong but that can be done as I have been able to add those units in, I also hope that maybe in the future if people are willing, the community can add more units like the M113 ACAV and other awesome units of the war but for now I'm only doing what's already available for me.
I'm also working on adding units for the NVA as well and in the future may make The ARVN and Vietcong seperate teams but that's another story.