I am not sure that the traditional notion of idealism precipitated American entry into the Vietnam War.  If there was idealism present, it was in the genuine and sincere fear of Communism.  The "domino effect" as well as other idealistic theories that suggested a real and viable threat in Vietnam going Communist helped to spur on the war effort.  I don't see the traditional notion of American idealism such as justice or freedom as entering the discussion without the understanding that stopping Communist spread in South East Asia was in American interests.  It is here where idealism was present in the entry to the Vietnam War.  One could argue that such a desire is not really idealistic, but in terms of assessing how there could be idealism in the American entry, I think that the fear of Communism and the envisioning of a world without it could be important elements to constructing this idealism.  At the same time, I think that it is important to suggest that many of the soldiers, especially at the start of the war, were trained to believe that the Communist threat is an intrinsically dangerous one, suggesting that even if this is now viewed with cynicism, at the time, it was viewed as an idealistic conception behind the war effort.