A thorough treatment of the Iraq/Vietnam analogy can be found here.
Choice bits:

"From a strategic perspective Vietnam was considered important only (okay, primarily) because of the domino theory (one nation falls to Communism, all in the region will). One of the theories used by the administration to sell Iraq was that it would function like the domino theory in reverse..."

-snip-

"The simple fact is that a guerilla war is being fought in Iraq, that the US was misled via an utterly ridiculous ideology and a lack of hard facts into the conflict; that US ignorance of the situation before, during and after our arrival on the scene was and is boundless; and that the US is bogged down in Iraq for as long as anyone can foresee with a steady number of casualties reminding us everday of the quagmire, the avoidable quagmire, that the US government perpetrated both on Iraq, and on its own people.

Just like Vietnam. "