Amir Taheri counts the many differences
between the two conflicts (story via Banafsheh Zand-Bonazzi email), and cites one way in which the two coudl resemble one another:
Having said all that Iraq may yet become "another Vietnam" in only one sense. The Americans may still decide to snatch defeat from the jaws of military victory just as they did in Vietnam. There are many in the US political and cultural elite who want Iraq to fail with an almost pathological ardour solely to get at George W Bush and his supposed cabal of "neoncons". A Democrat controlled Congress could cut the budget for the US troops in Iraq, forcing their withdrawal.
He doesn't foresee that such an outcome would lead to a repeat of the Fall of Saigon, at least not at the hands of the al-Qaeda and Saddamite insurgents:
A precipitous US withdrawal from Iraq is certain to complicate matters for the newly created democratic system.
But it would not mean a seizure of power by Al Qaeda and the Saddamites. The terrorists and the insurgents could continue killing people and causing mayhem for many more years just as their counterparts did in Algeria, Egypt and Turkey among others. But one thing is certain: Al Qaeda and Saddamites will never rule in Baghdad.
There is no Ho Chi Minh, but there is Iran, who may threaten a fragile Iraqi state if Western troops disappear. We already have evidence of Iranian-built munitions being smuggled into Iraq (see also here). If the coalition forces leave, would the Iranian flag soon fly over the Shi'ite portions of Iraq?