in regards to the domino theory, I think it was true even then. The Vietnamese war in Cambodia did happen and it was a disaster for Vietnam mostly because of the high losses the Vietnamese had sustained from fighting the French and especially the Americans.
Had North Vietnam beaten the South swiftly and quickly with minimum loss of life, the spread into Cambodia could have even succeeded. Remember that there were communist insurgencies in Malaysia and Indonesia at the time too.
So it was valid then, and it's still valid now. At least that's what I think.