Domino theory was never totally dis-proved. Regardless of the fact that Vietnam eventually went communist, America showed great resolve over 4 Presidencies, both Republican and Democrat, in its attempt to "stop the rot". The fact that they did may well have had effects in both Peking and Moscow. If the US had walked away from Vietnam at the first sign of trouble its credibility in Europe might have been severely weakened. Would we be looking at a reunited Germany if Vietnam had never happened?
The theory is alive and well in modern politics. Yugoslavia had its anti-Serbian domino theory. The USSR sees Chechnya as the first of several potential dominos and they also see fundamentalist Muslim ideas affecting several of their former autonomous republics. There are also those in South America who never saw Castro as an real problem but who have a more concerned view of Chavez.
I wouldn't be in too much of a hurry to consign this theory to the dustbin just yet.