No argument that Castro has achieved a lot more than Gulbaddin Hekmatyar, and the USSR did tend to support more progressive movements.

Glad we got that straight :-)  Then you go and spoil it:

 But in strictly cold war terms the USSR support for Castro (in the US sphere of influence) was no different from the US support for Gulbaddin Hekmatyar - in the USSR sphere of influence.  He was meant to attack the Russians.  He actually killed more afghans.  Guess who had the more incompetent foreign policy at the time.

Whose definition of "strictly" - it was not a technical exercise. The Cold War was political, it involved different ideologies and values - hence the bit you admitted above reflects the basis of the conflict - in a sense, what it was "strictly" about. The US was not concerned with freedom and democracy, but protecting and expanding the capitalist system and the gross social inequalities inherent in it.

See the interview with Brzezinski - he didn't think their policy had been incompetent, he didn't care what happened to the Afghans, the US succeeded in luring the Soviets into their own Vietnam.  Also the latter was not the "tragic error", as it is usually presented even by "liberals" in the US. It was, as Chomsky says, part of deliberate, consistent policy and it was a success:

Noam Chomsky: Well, I don't think that Vietnam was a mistake; I think it was a success. This is somewhere where I disagree with just about everyone, including the left, right, friends and so on.
...
the business world turned against the war and decided this is just not worth it. They said we have already achieved the main objectives and Vietnam is not going to undergo successful independent development. It will be lucky if it survives. So it is pointless; why waste the money on it. The main goal had been achieved by the early seventies.

You start reading in the Far Eastern Economic Review that this was a pointless enterprise, you guys have basically won so just go home and quit. Why ruin your economy, spoil your situation in the world scene and so on. And they assumed that now that it is destroyed it will sooner or later be absorbed into our system, which is in fact what happened. Well that's a partial victory not a defeat. The defeat was that they didn't achieve their maximal goal which was to turn all of Indochina into something like Guatemala or the Philippines, and that they didn't achieve, but they did achieve their main goal.

http://www.zmag.org/content/showarticle.cfm?ItemID=7143

As he also points out, the majority of the US population are less indoctrinated than the intelligentsia:

... [in] 1982, polls indicate that about 70 percent of the American population regarded the Vietnam war not as a "mistake," but as "fundamentally wrong and immoral." Many fewer "opinion leaders" expressed that view, and virtually none of the really educated class or articulate intelligentsia ever took that position. That, incidentally, is quite typical. It's typical for educated classes to be more effectively controlled by the indoctrination system to which they are directly exposed, and in which they play a social role as purveyors, hence coming to internalize it. So this degree of servility to the party line is not unique to this example. But the point is there's a split, a very substantial split, between much of the population and those who regard themselves as its national leaders. That is even given a technical name -- it's called "Vietnam syndrome."

Notice the term, "syndrome," as applied to disease. The disease is that a lot of people are opposed to massacre, aggression, and torture, and feel solidarity with the victims. Therefore something has to be done about that. It was assumed in the early 1980s that the disease had been cured, and by reading the productions of the educated classes you could certainly have believed that. But in fact the disease was never very widespread among the educated classes. However, among the population, it remains widespread and it's a problem -- it impedes, it inhibits direct intervention and aggression.

http://www.chomsky.info/talks/19850319.htm

Thus, despite all the servile media coverage which did mislead most Americans, there was opposition to the attack on Iraq even before it happened, while with Vietnam it took years before there was significant opposition.

Maybe it's because I'm a Londoner - that I moved to Nice.