In short answer, the overall Vietnamese casualties during the border war waged by Pol Pot and his Maoist genocidal regime was enormous high. A lot of Vietnamese people were pillaged, raped and brutally murdered in a lot of massacres.
Thanks to the strong military supports from the Maoist regime in China, the Khmer Rouge army was fully-equipped with significant amounts of Chinese military hardware.
Khmer Rouge soldiers in action, circa 1975–1977.
With a lot of AK-47s, B-40s, Type 59, Type 63 tanks and J-6 aircraft, together with 23 infantry divisions and 200,000 troops under his command, from April 1975 to January 1979, Pol Pot was so overconfident to wage the bloody border war against Vietnam.
According to the memoirs of Mr. Lưu Văn Lợi as a well-known Vietnamese diplomat at that time, during four years of Pol Pot’s border war (1975–1979), Vietnam suffered:
“79,860 casualties, or 74% of the casualties in the resistance war against the French”.
(Lưu Văn Lợi, “50 Years of Vietnamese Diplomacy (1945–1995)”, page 376, The Gioi Publishers, Hanoi, 2006)
International correspondents at the site of the Ba Chúc Massacre, April 1978.
One of the most notorious massacres caused by Pol Pot and his Maoist “comrades” in Vietnam was the Ba Chúc Massacre (18 April 1978), which costed the lives of 3,157 Vietnamese civilians.
This massacre also marked the beginning of the downfall of the Khmer Rouge regime in Cambodia, because the Vietnamese government subsequently passed the military solution to overthrow the bloody regime of Pol Pot and his men.
Hope you readers will know more about the high casualties that Vietnam suffered during this bloody war waged by the Khmer Rouge regime in Cambodia. Once again, thank you very much for reading my answer and best regards,