I have no idea if that is a sarcastic comment but I'll give you the benefit of the doubt..... He has been blackmailed into it? Despite landing a Duchy less than a week ago he quite happily falls in with a one man faction? That only proves my point that it makes no sense. Fact is, there is zero Vassal-Liege loyalty modifiers. What is the reason for positive relations if they can just be "blackmailed"?
Well the vassal might adore you, but he got
blackmailed. He doesnt fall in happily as, well he has been blackmailed (being pressed/extorted into the faction).
So you can use your spymaster in return to make him understand that whatever he had been blackmailed for won't matter, as long as he leaves the faction again.
... How many nobles were executed or families disinherited for leading or involved in rebellions? Hundreds. But here it makes you a tyrant......
You are aware that, after their defeat, you can rip each rebel off one title without tyranny ?
Executions didn't happen as often in the time period as you might think and even if you do so, incuring tyranny hasn't such a bad effect. Best to do when you are old and succession will happen probably soon.
In general:
High diplomacy atttributes and proper traits help, for that educate your heirs yourself.
Take care of problematic vassals beforehand.
Be aware of vassals with claims and the ambitious trait and take care of them before succession.
Hand out 20 gold after each succession of your dukes (works all the time the first few days as long as they didn't have income before) to improve relations cheaply.
Have a proper retinue and keep your own troops out of war (see faction strength calculation).
Take care of dejure requests by your vassals and anything you can do about maluses displayed in the relations tooltip.
Also, wrong thread to discuss this.