There are differences of course -- no two wars are alike -- but the potential parallels between Vietnam and our possibly-escalating war in Afghanistan are troubling enough to be a little eery. Whether you saw the VC as an indigenous resistance or the tool of the nation-state to the north, it was a force that was not about to go away and its leaders figured the U.S. would be eventually. Same with the Taliban, though it doesn't seem to have so obvious an outside support beyond havens in Pakistan (though elements of the Pakistani ISI may still be involved). The South Vietnamese government may not have been as hopeless as it was sometimes pictured, but it was shaky and corrupt, as is the Karzai regime. The drug trade didn't play a big role in financing the VC, as it is for the Taliban.
Most significantly, as this Register editorial points out, the core U.S. interest in Afghanistan is making sure al-Qaida, which is weaker but still likely has international ambitions, doesn't establish operating bases there. It doesn't have them now (most people believe), so we should make it known that whoever runs Afghanistan knows that if such bases come into existence we will blow them to smithereens, so please don't let them get established. Declare victory and get out.
I don't expect the U.S. to do that. Defense spoksemen say escalation isn't a sure thing -- McChrystal is due to issue recommendations in a few weeks -- so maybe there's hope. But I wouldn't bet on it.