Tip: When landing claimants of a different culture to press their claims, make them bishops.

I learned this trick in my current Wales game. Say you want to add a county or duchy to your realm, and you've got courtier in your realm (or willing to come to your realm) who has the claim you want...but is the wrong culture.

If you land them with a barony/county and press their claim the old fashioned way, now you've got a more powerful vassal who has a wrong culture penalty. They also won't do anything to help flip the culture of the county(ies) you just took to your own.

While you CAN educate their heir (if their heir is still under 16) and HOPE you get lucky enough to flip their culture to your own, there is a more surefire way to do it.

Give the claimant a bishopric. This will make him a bishop. Go and press his claim. When you win, he'll become a Prince-Bishop or Prince-Archbishop (depending on if you took a county or a whole duchy).

He'll still be a more powerful vassal of the wrong culture, but because he's a religious leader, you can appoint his successor to be someone of your culture. If you have a spare son, you can even name him as heir...which will kill four birds with one stone: Making the vassal your culture, putting a dynasty member in a position to earn the dynasty more prestige, getting rid of an unlanded son penalty and getting rid of a pretender since being a bishop disqualifies him from succession. Even if you don't have a spare son, you can always cherry-pick a content character of your culture to make for a well-behaved vassal.

Needless to say, this requires free investiture, which most players stay in anyway.

The only downside to this is that it creates a bishop vassal rather than a feudal vassal, who's levies and troops might vanish if they like the pope more than you. Making sure their heirs are content can help prevent this as that +50 content bonus applies to you but not the pope.