I'm going to make this fairly specific, but it's a situation that will probably be familiar to many people. I'm a king (The Petty King of Ulster, used to be the Earl of Ulster) and after my first wife died I married the Duchess of Normandy. To achieve that I got betrothed to her, and then had her older sister killed. We then had one daughter before she died. Her liege lord is the King of England (one of my allies).
My idea is to marry one of my grandsons (from my first wife) to my daughter, the Duchess of Normandy. Will their son become my vassal rather than the vassal of England? Because this is my goal with the whole thing. I really want to expand my kingdom outside Ireland.