You can look at the list of combatant.

Soviet ships in the South China Sea gave vital early warnings to Viet Cong forces in South Vietnam. The Soviet intelligence ships would pick up American B-52 bombers flying from Okinawa and Guam. Their airspeed and direction would be noted and then relayed to COSVN, North Vietnam's southern headquarters. Using airspeed and direction, COSVN analysts would calculate the bombing target and tell any assets to move "perpendicularly to the attack trajectory." These advance warnings gave them time to move out of the way of the bombers, and, while the bombing runs caused extensive damage, because of the early warnings from 1968 to 1970 they did not kill a single military or civilian leader in the headquarters complexes

The Soviet Union supplied North Vietnam with medical supplies, arms, tanks, planes, helicopters, artillery, anti-aircraft missiles and other military equipment. Soviet crews fired Soviet-made surface-to-air missiles at U.S. F-4 Phantoms, which were shot down over Thanh Hóa in 1965. Over a dozen Soviet citizens lost their lives in this conflict. Following the dissolution of the Soviet Union in 1991, Russian officials acknowledged that the Soviet Union had stationed up to 3,000 troops in Vietnam during the war

Some Russian sources give more specific numbers: Between 1953 and 1991, the hardware donated by the Soviet Union included 2,000 tanks, 1,700 APCs, 7,000 artillery guns, over 5,000 anti-aircraft guns, 158 surface-to-air missile launchers, 120 helicopters. During the war, the Soviets sent North Vietnam annual arms shipments worth $450 million.[299][300] From July 1965 to the end of 1974, fighting in Vietnam was observed by some 6,500 officers and generals, as well as more than 4,500 soldiers and sergeants of the Soviet Armed Forces. In addition, Soviet military schools and academies began training Vietnamese soldiers—in all more than 10,000 military personnel.[301]

The KGB had also helped developed the signals intelligence capabilities of the North Vietnamese, through an operation known as Vostok (also known as Phương Đông, meaning "Orient" and named after the Vostok 1).[302] The Vostok program was a counterintelligence and espionage program. These programs were pivotal in detecting and defeating CIA and South Vietnamese commando teams sent into North Vietnam, as they were detected and captured.[302] The Soviets helped the Ministry of Public Security recruit foreigners within high-level diplomatic circles among the Western-allies of the US, under a clandestine program known as "B12,MM" which produced thousands of high-level documents for nearly a decade, including targets of B-52 strikes.[302] In 1975, the SIGINT services had broken information from Western US-allies in Saigon, determining that the US would not intervene to save South Vietnam from collapse

In 1950, China extended diplomatic recognition to the Viet Minh's Democratic Republic of Vietnam and sent heavy weapons, as well as military advisers led by Luo Guibo to assist the Viet Minh in its war with the French (1946–1954). The first draft of the 1954 Geneva Accords was negotiated by French prime minister Pierre Mendès France and Chinese Premier Zhou Enlai who, seeing U.S. intervention coming, urged the Viet Minh to accept a partition at the 17th parallel.[292]

China's support for North Vietnam when the U.S. started to intervene included both financial aid and the deployment of hundreds of thousands of military personnel in support roles. In the summer of 1962, Mao Zedong agreed to supply Hanoi with 90,000 rifles and guns free of charge. Starting in 1965, China sent anti-aircraft units and engineering battalions to North Vietnam to repair the damage caused by American bombing, man anti-aircraft batteries, rebuild roads and railroads, transport supplies, and perform other engineering works. This freed North Vietnamese army units for combat in the South. China sent 320,000 troops and annual arms shipments worth $180 million.[293] The Chinese military claims to have caused 38% of American air losses in the war.[24] China claimed that its military and economic aid to North Vietnam and the Viet Cong totaled $20 billion (approx. $143 billion adjusted for inflation in 2015) during the Vietnam War.[24] Included in that aid were donations of 5 million tons of food to North Vietnam (equivalent to NV food production in a single year), accounting for 10–15% of the North Vietnamese food supply by the 1970s