So I was playing as the duke of Brittany and somewhere along the way my heir was taught by an Occitan and he adopted that culture. Now I didn't notice before he ascended to power, so I just chose to roll with it. And because of it, I tried to replace all my Breton vassals with Occitan noble men I invited (this was around 1090). Fast forward 70 years and all my vassals were now of Occitan descent, but my people remained Bretons. I am wondering if there's anything I can do to speed up the process or if it just happens at random.