The same people who considered Vietnam "winnable" in 1968 are the same type of people that consider Iraq "winnable" now.
For the last year of LBJ's Presidency his Secretary of Defense was Clark Clifford. He came in when McNamara stepped down. Secretary McNamara, one of the Vietnam War's key architects & a cheerleader for the first several years, could no longer hold onto his sanity and claim that Vietnam was 'winnable' at the same time.
While Rumsfeld & McNamara are almost eerily similar (right down to the matching glasses & haircut) they difer greatly when it comes from learning from one's mistakes.