How to get my own castle

So I was wondering if you guys might be able to help me I've been trying the last two days after buying the game to capture my own city or castle. I sort of am specifically looking to get an empire or Vladian settlement, as I like their troops best.
The problem is that I can't seem to stop the massive armies from turning up. I have 60ish cataphract/banner knights and another 40 units on the various tiers below them. I can beat armies 2-3 times my size but Douchebert (the Vlandian king) keeps showing up with and army of 500+ which is just too big to take on.
I dont really want to be anyone else's vassal. I want my own empire, not to be a knight to another emperor. I couldn't find a way to take over as king of the Vlandians in my other playthrough so I assume thats not possible too? Plus when I'm someone else vassal they end up giving me my first castle in a culture I dont care to build an army out of.
Any help is greatly appreciated!

Edit: I should also probably say I have something like 10 vladian lords as prisoners I'm dragging around right now, and I'm burning every village I pass, but this doesn't seem to slow Dergbutts ability to field an army.

submitted by /u/Frostfangs_Hunger
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