The nations of Laos, Vietnam and Cambodia were already directly involved in the series of civil wars classified under the Vietnam war, so I'm going to ignore them for now.
The Vietnamese War lasted until 1975, and the Thai Communist Insurgency occurred during that period; Thailand was also engaged in the war as a supporter of South Vietnam, regardless. Consequently, a Thai People's Republic could also officially enter the war against the Kingdom of Thailand, bringing a sixth nation into the war on North Vietnam's side (Or at least, a sixth Rebel faction.)
A similar communist insurgency occurred inside Burma (Modern day Myanmar), with the White and Red Flag Communists supported by China fighting the Union; The White Flags were mostly concentrated in the Rakhine State and therefore the war could engage three more members (Burmese Socialist Republic and Democratic Republic of Arakan on North Vietnam's side against Union of Burma on South Vietnam's side.)
Bangladesh's communist rebellion happened merely three years from the end of the war IOTL, but if the three sides joined it could likely extend the war, especially with all the additional combatants. The Marxist-Maoist Factions would likely merge to fight against the Republic initially, and additionally would presumably extend the conflict into one of liberation for the Rohingya people too, further pushing against the Union of Burma.
Malaysia's communist insurgency also occurred in the war's timespan, with Indonesia directly supporting the Communist Rebels. The reason Indonesia and Malaysia didn't merge IOTL is (well one reason is) Indonesian riots against the Malayan Sultans (influential people in Malaysia), which impaired relations between leaders. If Indonesia hadn't committed mass murder against the Communists during the 1965 purge, or even if they had become militant in reaction during that year, they could have potentially thrown Indonesian into civil war as well. The PKI (Indonesian Communists) might merge with the Malayan National Liberation Army, thus leading to a united People's Republic of Melayu Raya in a war against both the Malaysian Federation and Indonesian Republic.
Philippines is also obviously famous for their Communist insurgency, so they could easily also be engaged directly in the war (With a rebellion on the North Vietnamese Side), as the People's Republic of Luzon and the Communist Republic of the Philippines could easily declare independence and fight the conflict, eventually presumably merging into one Communist Coalition (As the CRP previously did.)
Regardless, such a war would decimate the entire region and be extended for who knows how long compared to our timeline...