Mao was as responsible for American deaths in Vietnam (since he was funding/arming the Viet Cong) as the Ayatollah Khomeini was for American deaths in Lebanon at the hands of Hizbollah, so again, why was Nixon's negotiation with Mao OK but any dealings with Iran are not? If anything, the latter was worse, because with Iran today, we're not dealing with Ayatollah Khomeini, whereas Nixon was dealing directly with Mao, as opposed to a less psychopathic successor.
I agree that Obama isn't the right person to negotiate with Iran (or with any other country, friend or foe), but that's a separate issue from whether it can or should be done.