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Click on the province and click "Grant". It will show a list of vassals you can grant the title to. One of them should say "- Promised Kokujin title" after their name.

Note that, if there's more than one, you can only fulfill one promise; the other promises will be broken and the characters will be upset for a while.
 
Click on the province and click "Grant". It will show a list of vassals you can grant the title to. One of them should say "- Promised Kokujin title" after their name.

Note that, if there's more than one, you can only fulfill one promise; the other promises will be broken and the characters will be upset for a while.
Thanks, I didn't realize that it would tell me when it was time to grant.
 
Also note that vassals will ask for the next kori or daimyo title even if you've already promised it to somebody else. If you agree, you will automatically break your promise to the first vassal, and incur a malus with his loyalty.

It is a good idea to check this screen, then, before agreeing to any requests when they pop up. (Or just routinely rejecting them to keep your hands free, as I do.)