There is a CMSF mod called "Heaven & Earth" that mods the terrain to that of an "unspecific" SE Asia that could be Vietnam. From the manual:
Heaven & Earth is a supermod centring on the fictional country of Bong-Hai, the Bong-
Hai civil war & its messy aftermath.
The Bong-Hai Civil War is suspiciously similar to the real life Vietnam War but, for
technical reasons (architectural, floral, TOE, linguistic, vehicular & weaponry), because
the team could not truly represent that conflict we decided to go the fictional "Vietnamesque"
route.
The two sides are...
• The People's Army of The Free Democratic Republic of Bong-Hai & its People's
Militia insurgents (aka "The Roundheads")... they are the Earth component of this
narrative & are as similar to the NVA & VC as we can get them.
• The Imperial Army of the Divine Kingdom of Bong-Hai including its iconic, M16
wielding Imperial Cavalry (aka "The Cavaliers") .. they are the Heaven
component of this narrative & represent both the poorer & better performing
elements of the ARVN.
The Heaven & Earth modpack is designed for CMSF2 version 2.03 or 2.04, it is advised that
you use no other mods alongside Heaven & Earth as it is a complete package with a unique
UI, sounds, scripts, special effects, vehicle's & weapons... many of which are swapped
internally by the use of tags.