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I think every time a ruler change occurs the vassals get an event to either
A) recognize the ruler
B) break from the realm

Also vassals get hit with a 20-25 loyalty drop automatically (not sure on the number). It won't help you that your heir has low prestige; a good way around this is to give your heir land so he can build up prestige before his inevitable assention.

As to having a vast realm, the more vassals you have, the more potential rebellions.