I suspect that given our national experience in the past few years in Iraq and Afganistan, the old but somewhat accepted myths about the Vietnam War with respect to immorality (i.e. civilian casualties including babies, immoral SVN government and US ruthlessness) are more and more falling on deaf ears.
Put simply, the public should be more familiar with the difficulties of fighting an enemy that hides amongst civilians and places little value on the lives of their individual troops. Indeed, in current times the enemy appears to place more value on their deaths than their lives.
Moreover, the nortion that the SVN government was immoral and thus, so were we for supporting them seems more and more silly. We allied with the USSR before Vietnam and have allied with Iraq and Afganistan recently; none of which seem particularly moral. We established such alliances because it was in our best interest and standing alone, that is no substantial basis for a moral judgment. As subsequent events demonstrated, the alternative by way of standing idle was worse.
As a sidebar, the Frontline show last night reporting on the operations of the enemy in northern Afganistan illuminated the need for local support. These guys wouldn't be able to operate if the locals were against them. It will be interesting to see how the current clear and hold offensive in the south turns out - whether we can gain local popular support.