I think that the Iraq war is perhaps worse than Vietnam because it's really a war waged mainly due to circumstances being favourable, whereas in Vietnam there was a discernable strategic rationale (domino theory, containment, rollback). I think that the war in that sense mimics the war of 1812. Of course in Vietnam there were lies used as a pretext, the strategic rationale was flawed and domestic politics took over the war after a short while. Will the world be less able to forgive a war fought for no reason at all? Could be.

Another problem is that America is not fighting a clearly ideological entity. In Vietnam, the enemies were Communists. Or so the narrative goes. Currently the enemy is Al Qaida, Baathists, Islamofascists, sometimes Sunnis, sometimes Shi'ites, 'rogue states', etcetera. A lot of these are ethnic and religious categories. This may lead to a longer memory.

On the ground, Vietnam was still worse than Iraq. But there was no official endorsement of torture in Vietnam, which also changes the story of the war. And there are plenty of opportunities for Iraq to escalate even further, unfortunately.