SOURCES:

https://www.globalsecurity.org/military/world/vietnam/hist-2nd-indochina-ussr.htm
https://alphahistory.com/vietnamwar/chinese-and-soviet-involvement/ 
https://www.rbth.com/blogs/2015/04/30/vietnam_war_the_critical_role_of_russian_weapons_42917 
https://www.quora.com/Did-Soviet-troops-ever-see-direct-combat-against-US-Forces-in-the-Vietnam-War
https://thenewsrep.com/53782/war-vietnam-bigfoot-soviet-military-experts/ 
http://csef.ru/en/oborona-i-bezopasnost/504/shest-rassekrechennyh-operaczij-gru-genshtaba-vs-sssr-7540
http://acepilots.com/vietnam/olds_bolo.html
Review: Russia's Vietnam War
Reviewed Work: The Soviet Union and the Vietnam War by Ilya V. Gaiduk, Review by: Jeffrey Kimball, Reviews in American History, Vol. 25, No. 1 (Mar., 1997), pp. 157-162