The Vietnam War may have been winnable but only at a great cost in American and Vietnamese lives. These lives greatly outweigh what the fruits of victory would have been, as the Domino Theory proved wrong.
Attacking Kerry for being credited by the Vietnamese for helping them win the war is just like attacking George W. Bush for creating terrorism through his policies in the Middle East. While they certainly do not credit Bush personally, some credit his policies for causing angry young people to take arms up against America. Kerry is as much for communism as Bush is for terrorism. I am not saying that Kerry's anti-war demonstrations and Bush's anti-terror campaign are comparable overall, only the fact that they had possible unintended consequences, North Vietnam's faster sweep of South Vietnam and greater recruiting for terorist organizations respectively. Do not read the point of my argument as that we should blame Bush for terrorism, but take it as that Kerry can be blamed as much for North Vietnam's victory as the President could be held for increasing terrorist recruits.