There is no one-best-recipe.
Your vassals should never have too many titles - in this case they will have power to singlehandedly overthrow you and usurp your title or make you to lower crown authority or taxes or levy size. And be sure - there will be time when they want it.
Also don't give titles to someone who hates you - in this case make them your vassal's vassal - this is a suitable solution for unlanded sons or brothers. It also makes them harder to raise war for your crown, because they should firstly became duchies, and this involves war with their liege, whom they may hate no so strongly.
Give titles to ppl who have Content trait (+50 relations), to someone with same sins and virtues as you, also try to make your king as good as your can - virtuous, kind, just, prestigious, pious.
Look for vassals' background - don't give land title to priests and mayors - they will have "Wrong government type" penalty.
Respect - is everything. You can fight with rebel vasslas, but not with all of them at the same time.
Hope they will be helpful!
The "Revoke the County of ___" plot only requires that a count level vassal hold more than one county (or that a duke holds a county outside of his duchy) and that you not be incapable or an imbecile. BUT meeting those requirements does not ensure that the plot will show up. There is also an apparently random element in which plots appear and that random element is affected by a number of personality traits and other factors. So a "Content" character is very unlikely to see this plot. And a "Kind" or "Slow" character is somewhat unlikely to see it. But an "Ambitious," "Decietful," "Envious" character who meets the basic requirements will almost certainly see it. The chances of it appearing are also affected by your opinion of the target. If you hate them (according to the game's calculation of your opinion) then it is more likely that you will have this plot available.
At least that is how I read the files. I'd welcome anyone who has more insight into this.
Best Answer
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Choose your Casus Belli wisely. You must have chosen a casus belli which retains the vassals of enemy in case of your victory. I have the horselord DLC but I haven't yet played it (Just had a test run, didn't quite like it). Anyways, to answer your question, no you can't just do that.You either have to manually revoke all the titles. Or you could write a custom event which would be triggered as a result of a successful invasion to revoke titles of all vassals which are not from your culture group (Which would be your newly conquered vassals, to be precise but I am not sure if it is possible to revoke ALL titles held by a vassal. Event could work for primary title at least). But that would be modding. In original game, nopes, you can't.