Vietnam was a loss/defeat no matter what you call it or try to justify it. We joined the side of the South to fight the North and they outlasted us/beat us. It doesn't matter why we pulled out or left the country, we chose to do so and our ally, the South, lost. The corruption/ineffective government in the South would not have matter if we won the war. It is true that we lost the war due to public opinion and politicans preventing the military from doing their jobs and just rolling over the North (yes, partly to keep the war contained to Vietnam and not starting open war with USSR at the time). We lost, no different than a kid losing at football and taking their ball and going home before the game is over and claiming they didn't "lose".
Do you consider Vietnam a victory on our part or a tie? Do you consider Afghanistan a defeat?