Is there any way to influence where your vassals capitals are?
If you give an unlanded courtier a county, then a higher rank title and any amount of titles below it, they will usually keep the first county as their capital, even if it not in the de jure realm of their primary title. In this way, I've kept capitals of far away kingdoms near my own capital.
Does this strategy work long term?
Best Answer
I believe non-player characters will only choose to move their capital province to the historical de jure capital of their primary title. All de jure kingdoms and duchies have a de jure capital, along with some titular titles. Apparently the AI will even go so far as revoking the title of the capital province from a vassal if necessary to do so.
I'm not sure what impact this would have on your strategy because I'm not sure of what benefit exactly you're trying to achieve by controlling where your vassals have their capitals. I think the only way you can limit the movement of a vassal's capital is to either ensure either you or the vassal owns the de jure capital province. Otherwise your vassal is likely to go to war eventually to get it.