@Ron_C, in addition to what @tedd said, there are several other reasons:
• The North Vietnamese did not give shelter to a terrorist organization that murdered 3,000 of our citizens.
• The Taliban did not enjoy anywhere near the popular support that the Viet Cong/North Vietnamese. Nor did our enemy in Vietnam wage campaigns of ethnic cleansing against religious minorities in Vietnam.
• The ideology of the communists in Vietnam is nowhere near as repugnant as the ideology of the Taliban. I’m sure you don’t need me to go into details.
• There is no military draft for Afghanistan.
• You have completely exaggerated the civilian deaths of the war (and by the way, Afghans are not middle easterners). In Afghanistan, 14,000 – 34,000 civilians have been killed. In Vietnam, possibly 3 million civilians were killed. No civilian death is ever justified, but we are talking about a difference of several orders of magnitude. (The Iraq War is worse—up to 150,000 dead civilians—but still nowhere near the scale of Vietnam).
• The rules of engagement in Afghanistan, even during Bush’s insane mismanagement of the war, were far better than the ROE in Vietnam. In 2001, our forces were on the cusp of capturing bin Laden at Tora Bora. We should have, and the fact that we didn’t was a huge failure. But we also didn’t carpet bomb the region, like some military officials were recommending. Carpet-bombing was extremely common in Vietnam; so was the use of DDT and napalm. In general, our ROE in Vietnam allowed indiscrimate slaughter; even a cursory examination of the WikiLeaks Afghan files shows that our soldiers there are much more concerned with preventing unnecessary civilian deaths. And Obama, McChrystal, and Petraeus deserve credit for further constraining our use of airstrikes in civilian areas and other lethal uses of force; today Taliban account for 85% of civilian deaths in Afghanistan.
• I don’t have a problem with “imperialism” in an abstract sense. I would be fine with a legitimate world police. I think the state of affairs in human society is a set of overlapping power structures, some more abusive than others. The “empire” is simply the largest Russian nesting doll of such power structures, and if the empire is less abusive than smaller “dolls,” then I have no problem with the empire preventing abuse. Of course, the US imperialism (like past instances of imperialism) is often abusive, is often poorly or callously deployed, etc. But in terms of Afghanistan; we are talking about a country that is a failed state, a nightmare for more than half the population living there (women, religious minorities), that is basically in a state of perpetual war—in no small part because of the US’s footprint there during the Cold War. If an “empire” can help stabilize this region, by essentially acting more like “police” and less like carpet-bombing military forces, I have no problem with that.