Couple of quick questions:
My son had a claim for the kingdom of Sweden so I pressed the claim, beat the swedish king and my son got the throne, but he didn't become a vassal king. It says in the declare war option that if the claimant is of my dynasty he will become a vassal. Does that not count for kingdoms?
Also, I was the current king of Denmark and Norway and tried giving my other son the kingdom of Norway cause I thought it would be cool to be King of Denmark with my sons ruling Norway and Sweden, but he became independant too. That wasn't my plan. Isn't it possible to be the liege of kings? Is there a way to become an empire or something to make it happen?
I think the tooltip is misleading. A king can never be the vassal of another king (likewise, a duke will never be the vassal of another duke, etc.). I believe if you were an emperor, it would have worked out differently. There is no way to form your own empire, you'd have to take one of the current ones.