Let's stipulate Vietnam was a civil war. So what? There were certainly good guys and bad guys, and let the record show the bad guys won, which was not in our interests. This in turn led to many humanitarian calamities. And, recall, another superpower intervened in that civil war, and it worked out pretty well for the Soviets.
More to the point, it's ludicrous to believe America has no interest in who wins or loses various civil wars, including Iraq's. The 20th century would have been a lot more pleasant if the Bolsheviks had lost the Russian civil war, and the 21st will be a lot more ugly if Sunni Salafists or Iranian pawns win in Iraq.
I'm not saying a civil war is a desirable environment for anybody. But nor is it a geopolitical black box absolving all concerned from moral and strategic discrimination. And yet that is exactly what advocates for withdrawal from Iraq want everyone to believe, but only when it comes to Iraq.