A lot of what went wrong in Vietnam was not Westmorelands fault, but there is one area where his strategy was mistaken, and from which it would seem we are applying the lessons in Iraq. Westmoreland--and the other service chiefs--were fans of the "big unit" search-and-destroy war in the mid-1960s, which is why total troop levels rose to over 500,000 by 1968. All of that time very little was being done to train the Vietnamese army to fight for itself. (McNamara rejected this, saying by the time the Vietnamese were trained, wed have the war won.) When Gen. Creighton Abrams took over in 1969 and began the drawdown of troops, things went much better because Abrams largely abandoned the search-and-destroy strategy in favor of an "enclave" strategy that emphasized turning over the war to the Vietnamese. (See Lewis Sorleys fine book, A Better War, for a full account of the Abrams command.)
So when you hear people complain that we dont have enough troops in Iraq, remember that the opposite complaint was made about Vietnam. From afar it seems our basic strategy in Iraq is the right mixture of Westmoreland and Abrams--protect crucial enclaves, train the Iraqis, and engage in specific search-and-destroy missions (Falluja, etc) when key targets can be identified.