It is highly doubtful that Vietnam and Korea were more capable of resisting Chinese attempts at assimilation if for no other reason than the geography involved.
The first attempt at assimilation occurred during Mongol rule of China. The Mongols had some success in Korea but utterly failed in Vietnam, as they were ill equipped to fighting in a jungle atmosphere. They were accustomed to fighting on horseback on the prairies; neither they nor their horses were could handle the humidity and jungle conditions, and they were forced to abandon the attempt.
Japan was saved from conquest when Ghengis Khan sent huge fleets of ships to attack Japan, but which were destroyed by typhoons. In 1281 Ghengis sent a fleet of 5000 ships with 100,000 men to attack Japan. This was the largest fleet assembled before World War II. A typhoon destroyed 450 of the ships and many men perished; the attack therefore had to be abandoned. The Japanese referred to the typhoon as a kamakaze, or "divine wind" which had saved them. This same name was adapted by suicide bombers in World War II in an attempt to revive Japanese nationalism.