Yes, you're right. And I know that many RVN units performed admirably during those terrible final days. But IMHO, the analogy still holds. Afghanistan will end up exactly like Vietnam did.
Well, not exactly. After taking South Vietnam, the North did not then use it as a base to attack the US. After taking Afghanistan, radical islam WILL use it as a base to attack the US (again).
I'm actually cautiously optimistic. Because of Kashmir and Central Asia, both the Indians and the Russians have a reason to keep the Afghan government standing. It all comes down to funding. The Soviets sent billions in brand-new equipment on credit* to the North Vietnamese just prior to the end in 1975. Russia alone has a bigger economy than the Soviet Union, thanks to free market reform and skyrocketing commodity prices. Bottom line is that Russia + India > any combination of private Gulf state donors.
* The mammoth scale of this loan was such that it never paid back in full and was written off against rent payments for Soviet bases in Communist Vietnam.