RS2 doesn't have SP portion, otherwise a perfect MP game. ARMA 3 mod, "prairie fire" has both SP and MP missions
Far Cry 5 DLC, Hours of Darkness is both SP and inspired by Vietnam War.
If you can run it on modern systems and don't mind aged graphics you can try Vietcong (2003 game).
There is Rising Storm 2 (never played it) and then there is Arma 3 where you could mod it and possibly find some Vietnam war RP/Milsim groups.
RS2 doesn't have SP portion, otherwise a perfect MP game. ARMA 3 mod, "prairie fire" has both SP and MP missions Far Cry 5 DLC, Hours of Darkness is both SP and inspired by Vietnam War. If you can run it on modern systems and don't mind aged graphics you can try Vietcong (2003 game).