As to Kerry, he DID have a huge impact on the war in Vietnam. Much greater than I think you believe. NOT by what he did in Vietnam, but by what he did here in the states after he got back.
The peacepukes in Berzerkeley and other leftist hotbeds around the nation were just a rag-tag bunch of Communist-incited hippies, who everyone was pretty much ignoring (...hoping they'd just GO AWAY) until Jane Fonda and John Kerry came along. Fonda's name-recognition and Kerry's apparent status as a supposed 'decorated war-hero' finally gave that bunch of losers, and their looney-left, blame-America-First supporters, and their chicken-little message, a bit of credibility with middle-America, not to mention the MEDIA attention, which they had not sucessfully captured before. That was the beginning of the end.
The war in Vietnam was not lost by our military in the jungles of Southeast Asia. It was lost on the streets of America, with people like Kerry and Fonda leading the (enemy) 'troops'. Why else do you think Kerry's picture is actually hung in a place of honor in the 'Viet Cong Hall of Fame' or whatever they call it???