I wouldn't conflate the communists in Vietnam (USSR ally) with those in Cambodia (Chinese ally). Vietnam fought wars with both of the latter countries.
Winning battles doesn't count for much if you don't meet your strategic objectives (if you even have any). The same story has played out many times since then; the Soviets dominated the Afghans in every possible way, but accomplished nothing of note. Iraq now is much the same.
Losses in "Afgan" (Russian slang name of the country) weren't too high (15k either killed or died from wounds), even compared to American losses in Vietnam (58k KIA), and technically there were no major battles lost. Vietnamese were backed by the USSR, Afgans - by Saudis and the US.
But from a political point of view it was a clear loss, similar to Vietnam for the US: no pro-USSR government established, anti-war movement back home, further deligitimisation of the Party, etc. Unfortunately for the Soviets, this strongly resonated with other problems at the time resulting in... The collapse.
Winning battles doesn't count for much if you don't meet your strategic objectives (if you even have any). The same story has played out many times since then; the Soviets dominated the Afghans in every possible way, but accomplished nothing of note. Iraq now is much the same.