Well, since this is in the topic of vassalization, how do you even get vassals without capitulation? Nobody ever asks me to be their masters, unless they're unstable and I refuse. As France, I took Algeria. England took Southwest Africa, near Mali. How come Mali wont ask me if they can be vassalized, but would take England?
Also, I don't have any cities in Asia or the Pacific, but how would anyone else in this case take Khmer as a vassal? I can probably conquer them with me power, but I don't want them to crumble.