The NVA were receiving the best from China and from the USSR, they had supply routes in other neutral nations and had a much better leader who had experience in Guerrilla fighting. Despite all the smoke and mirrors the best Ukraine is getting is hand me down weapons and a couple of the actual good ones.
Afghanistan was much more successful than this, we captured the important parts of the country quickly and had low causalities. Aside from some low intensity fighting, nothing much happened until we packed up and our puppet state decided to just not fight. This is not at all like that. Russia has not gotten the air control they need, have used up way more in terms of equipment and men, and still haven't gotten control of the country. Its true that afghanistan is far weaker than Ukraine, so a better comparison would be iraq. Take your pick of which war you want, we won them both very quickly, because our army is actually competent unlike Russias mercenary warbands.