I never actually use offer vassalization. It never worked for me, and sometimes rulers would reject my vassalization offer and then offer to vassalize a few months later. I think the probability of vassalization because you hold the Kingdom title is way too high at the moment. It seems to occur almost automatically when the old ruler dies. It's weird to watch Germany disintegrate down to one vassal and a couple provinces, only to have recovered almost all of its territory thirty years later.