Put Claiment on Throne, What to do now?

[Oaks]

Hello All,
As a duke (I think? Not the king, a step below) in Poland, just out of interest (still learning the nuances of the game) I invited some guy that had a claim to a small Irish dutchy to my court. I granted him some land so he became my vassel and then we lauched an invasion and won.
Now that it is under his control (and he is my vassel) I have a few questions:
1.Will his duchy still be Polish after he dies? Is independence his only way out of being ruled by Poland?
2.How can I put it in the hands of one of my family members, if not myself? I know I can revoke it or imprison and so on, but I wish to avoid the tyrant penalty (already a known murderer lol)
3. Do people generally use this technique (invite claiment, give land, press claim) for expanding their relm, or is it too much trouble or looked down upon?

I apologize for all the questions. I am playing ironman (I like the excitment and I would savescum nonstop if not for it) so I do not really have too much room to experiment with this situation.

Thanks in adance!

[Yaldabaoth]

I think you are confusing something here. If you are a duke, you can't have a duke as a vassal. You mean a count? If you indeed landed a guy and made him duke, you lost that land.
Anyway, yes it will remain under Poland. Exceptions are: Another title holder (of same or higher rank) under a different realm inherits or the ruler creates another kingdom title (I'm not sure if AI does this).
Just revoke it shortly before your ruler is going to die of old age. Your child will thank you. Or you could marry your son to a daughter of that ruler and kill all male offspring (check their inheritance law beforehand). If you were king, another possibility would be pissing him off so much that he rebels and declares independence. Win the war, he will be in your prison and you could revoke one title for free. But I don't think that works if you are a vassal yourself.
Anything you can do to grow is allowed.

[Oaks]

Thank for for the feedback. I always confuse the ranks when the game is not infront of me. I am not the king of Poland, but the ______ of Greater Poland. So I have the king above me, then my vassels (Dukes?) below me. My apologies for the confusion.

[Feynrik]

3 It is not so used technique to give land and then press claim. Instead I go for massiage alignment and by new generation taken over that subject countryparts. Or just plain and simple holy wars are also interesting.
Another thing is bothering me right now and I started a question thread about how come achievements stopped appearing even thou in Iron mode and no Mods and no gamers architect (or what it is called) and by some tricky events I do too have savescummed, but now none of the save files gives achievements. And I am sorry to write that once I got hooked with these all achievements I cannot longer play game without try meet the requirements for achievements. So, recommendation from me who knows how frustrating it is to play 30+ real hours to get that as Lombard y1300 achievement but still don't get it when passed that year. Really sucks!

[Oaks]

I am sorry to hear about your achivement probkems. Have you perhaps tried to uninstall and reinstall the game? Do you still see the little iron man fist when you load?