Hello there!

In a Soviet foreign policy memorandum circulated on January 13, 1967, by Soviet foreign minister Gromyko, it was stated that the Soviet Union would indeed assist North-Vietnam "in consolidating its defence capacity" to "repulse American aggression without getting directly involved in the war". The Vietnam conflict was of remote importance to the Soviets, who were wary of getting involved due to their near-military conflict with China during those years.

This policy remained unchanged, so in short, there were no Soviet troops fighting in Vietnam. There were advisers though to assist with the equipment sent by the Soviets, and particularly the KGB was active in providing Vietnam with signals intelligence on US targets. By 1965 the Soviets had already placed one of their trawlers near Guam to intercept US Air Force communications that gave away what was supposed to be surprise aerial strikes on the Viet Cong.

In 1968 the North Koreans seized an NSA vessel, the USS Pueblo, and shared its equipment with the Soviets. This enabled the KGB to obtain encryption keys of US military secure telephones used during the Vietnam War. The keys themselves were not shared with Vietnam, but any relevant communication intercepts were shared to aid the Vietnamese. The KGB continued to receive up to date encryption keys through its Walker spy ring within the US Navy that existed from 1965 until 1983.

The importance of this support should not be overestimated. Hanoi was not particularly impressed and occassionally repaid the favor by letting the KGB have American military equipment they captured or interrogate American POW's. On top of that, North Vietnam ran its own sophisticated signals intelligence missions. This was particularly easy as the US military consistently bypassed secure communications and encryption.

So in short, no Soviet troops did not fight in Vietnam. There were Soviet advisers and intelligence officers active however. Their importance should not be overstated I think. I hope this answers your question.

Consulted literature:

Anatoli Dobrynin, "In Confidence: Moscow's Ambassador to America's six Cold War Presidents"

Christopher Andrew and Vasili Mitrokhin, "The World was going our Way: the KGB and the Battle for the Third World"

Nigel West, "Historical Dictionary of International Intelligence"

James Bamford, "Body of Secrets: anatomy of the ultra secret National Security Agency"