I've heard him talk about his feelings about Vietnam quite a lot, including a recent interview on 60 Minutes. He got pretty emotional about it too.
Kerry's own website features a bio that is quite up front about it too:
By the time Senator Kerry returned home from Vietnam, he felt compelled to question decisions he believed were being made to protect those in positions of authority in Washington at the expense of the soldiers carrying on the fighting in Vietnam. Kerry was a co-founder of the Vietnam Veterans of America and became a spokesperson for the Vietnam Veterans Against the War -- Morley Safer would describe him as "a veteran whose articulate call to reason rather than anarchy seemed to bridge the gap between Abbie Hoffman and Mr. Agnew's so-called 'Silent Majority.'" In April 1971, testifying before the Senate Foreign Relations Committee, he asked the question of his fellow citizens, "How do you ask a man to be the last man to die for a mistake?" Sen. Claiborne Pell, (D-R.I.) thanked Kerry, then 27, for testifying before the committee, expressing his hope that Kerry "might one day be a colleague of ours in this body."