They mention in the article that the Ultimate Captain America of Vietnam is someone that everyone will recognize but who hasn't been in the Ultimate Universe yet.
Who do you think that might be?
John Walker? I think this the most likely person because he was such a flawed Captain America that grew up in the role to be disillusioned by the role of Captain America. He also comes from a very right wing background that gets twisted along the way during his tenure. I could easily see him as a sort of "Rambo: First Blood" type Cap.
Isaiah Bradly? He would be my second choice because an African-American Captain America coming out of the 1960s into Vietnam, then possibly being a Weatherman-like radical after his stint would make for an interesting story. An extreme left-wing Cap to go up against the very right-wing Ultimate Cap would make an interesting story.
William Naslund? Or Jeff Mace? Either would be interesting in that both were responsible (in MU time) of saving President Kennedy in 1946. Maybe in this timeline, one of them would go so far as to kill, maybe, Bobby Kennedy.
William Burnside? Another interesting character to play with...the radically conservative Cap that goes to Vietnam and breaks under the duress of that conflict.
My personal favorite: James Barnes. He could be the son of Bucky Barnes that becomes Cap after hearing his father's stories about WW2. Unfortunately, he becomes unhinged and it is now up to Ultimate Cap to bring in the son of his best friend down. This can add dimension to the conflict, because it becomes generational...a father/son dynamic of why WW2 was the "Good" war and Vietnam was the "Bad" war.
Vietnam Cap is the original caucasian Nick Fury, who was left for dead with his identity usurped by Ultimate Fury. ( I doubt they'd actually do this because of how racially loaded it is, nor would I want them to. Especially since Ultimate Fury has become outright evil and showing him next to Classic Fury would just bring home the point that the African-American version is the morally reprehensible one. But as long as we're wildly guessing...)