Ron, the Vietnamese won by outlasting America's will to fight. In effect, they won that war politically and psychologically rather than on the battlefield. And they deserved to, because the USA had no business being there in the first place.

There's more than one way to win a war or skin a cat.

The South could conceivably have won the Civil War in a similar fashion...politically and psychologically...if, for example, they had won decisively at Gettysburg (which they might have). War weariness and furious draft resistance in the North was the greatest threat Lincoln faced during that war. Had the South won handily at Gettysburg, Lincoln would have lost the coming election, McLellan would have been elected in his place, and McLellan had campaigned to end the hostilities. That would have resulted in the South maintaining its existence as a separate nation. The risk of further war between North and South at some point in the future would have been great, but the USA would at least for awhile have been split into 2 separate nations living in an uneasy peace.

That's why Lee went north in 1863. He never dreamed he had the material strength or manpower to conquer the North, but he did hope to break their will to continue the war if he could score a dramatic enough victory over the Army of the Potomac on northern ground. It was a calculated risk, a throw of the dice, and it failed. Lee handled most of his battles extremely well, but he was way off his mark at Gettysburg, mainly because a long string of previous successes had made him overconfident. He had come to believe his Army of Northern Virginia was invincible.