As I recall it's if they grow to have more than 50% of the population or land area of the master civ. Also if they drop to less than half the land area they had when they became a vassal, they break free as the master civ is clearly failing to protect them.
That's only for Capitulation. For regular Vassals, the Vassal can cancel at any time after 10 turns. Also, if the Vassal refuses any resource demand from the Master, the Vassalage is canceled and a war begins between them.
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