Here's the things I know for a fact based on mission area tags and real life counterparts.
Cambodia Areas: Sihanoukville, Ling Ho Spring, Thung Lung Cao, & Lumphat
Laos Areas: Attapeu, Nong Khiaw, Plain of Jars, MSS Leghorn, & Mu Gia Pass (Border pass between Vietnam and Laos)
South Vietnam Areas: Saigon, Da Nang, FSB Quan Loi, Phu Quoc, A Shau Valley, Da Krong, La Hot, A Lieng, Hue, Khe Gio Valley / FSB Khe Gio (Area is just south of DMZ IRL), & Paradise Island (SSPL name for Cù Lao Chàm)
North Vietnam Areas: Hanoi, Hai Phong, Ha Long Bay, Thud Ridge, & Lai Chau
Unsure but exists IRL: South Vietnam - Long Hai (IRL on the other side of the Mekong Delta), North Vietnam - Dao Phai (IRL between Hanoi & Hai Phong), Myanmar(Burma) - Kenglat / Khamkeut District, Bolikhamsai, Laos - Ban Kenglat (Both exist and Myanmar's is more well known and labeled on google maps)
DMZ Area: The DMZ has a visible shelled area south of Hai Phong and north of the Khe Gio Valley, however because it does not follow a river like it did IRL you can only guess past the visible area.
So far this map is my best estimate on the borders: