Slitherine

After several turns? 30 or 50?
Are there any other requirements?

It seems to happen in one way for the ai, and for the player, you will get gifts of provinces only. Non capital provinces bordered by several of your own provinces (I think 3) will eventually get gifted. Looks like very random as to when that happens.

I had one for more than 50 turns, he had two islands and gave me the rest of his territory, as well as paying me a pittance every now and again. Best way is simply to invade them - but there is a diplomatic cost, ie you get a minus on your diplomacy attempts for the way your have previously treated client states.