Soviet Union:  To what extent the Soviet Union played in Vietnam may never be fully known. They most certainly acted as instructors to the North Vietnamese Army and were highly useful in training it's Anti-aircraft units. Many high level PAVN officers were schooled in Moscow and spoke Russian. More info. on Soviet forces in Vietnam can befound: here.
    From 1954 to 1967; the Soviet Union spent a tad under 1.9 Billion dollars. From then until the wars conclusion the Soviets were sending $420 million annually. Most Soviet aid arrived via port Haiphong which was unbombed by American forces until 1972.
Cambodia: The government of Prince Sihanouk was officially "neutral" from the war's conception. However; 1963 he severed relations with the RVN and in 1965 the US. During the war Soviet war material was continually unloaded in Cambodia. Accounting for 80% of the supplies in use by NLF/PAVN forces in the southern most areas of the RVN. He also permitted vast hide-outs to be constructed on the Cambodian border. Even a small section of the Ho Chi Minh Trail was named after him. In 1970, his government collapsed and the Soviet Supply line with it.
Indonesia: Out of sheer spite to Malaysia; Sukarno welcomed a "permanent mission" from the NLF. As a result the RVN severed all ties with Indonesia.
Algeria: Having just concluded it's own colonial war with the French; showed it's support for the communist bloc by accepting an NLF delegation as a "permanent mission"
PRC: The largest player in communist efforts in South East Asia. Claims to have had 350,000 personnel stationed in North Vietnam during the war. China ofcourse maintains that these were all support personnel but; according to Blackjack-33 some Chinese soldiers served as mercenaries in direct ground combat. However; China's greatest help to the PAVN/NLF was in economic and military aid. The exact total is hard to pin-point but; was the approxiamete equivalent  of 2 Billion US dollars. Chicom copies of Soviet weapons were also contributed to the PAVN/NLF efforts 
North Korea: In a recent visit to Vietnam; the Foregin Affairs Minisier of North of DPKR Korea acknowledged that North Korean pilots had been involved in the war. He refused to disclose the number. While their he visited the graves of some 14 North Korean pilots.

Sources: Area Handbook for South Vietnam(April 1967), Blackjack-33, Associated Press Newswire, Inside the the VC and NVA.