The Kremlin warned that American support for Ukraine could turn into a decade-long folly, urging the U.S. to not oppose its invasion of the country as Congress appears set to pass a $60 billion aid…
The difference is that Vietnam and Afghanistan were civil wars. It's very difficult to win a war when a sizable portion of the citizens are fighting against you. Ukraine seems to be very united against a common enemy: Russia.
if anything, even if Russia were to magically take Kyiv tomorrow, they are going to be fighting heavy insurgencies for decades to come. it'll be the Afghanistan war (the one wot killed the USSR) 2.0 on caffeine.
And they are no where near as economically or militarily strong as they were when the USSR was even at its weakest