Well, what happens to the vassals after a war, depends of the casus belli used - holy wars, direct claims, ect. - will give you land without requiring to deal with vassals. Invasions do the same. If you're going around 'conquering' in such ways, you should get the lands for yourself. If your wars are for other reasons, you'll have to deal with the leftovers (the vassals). Still, revoking titles isn't too bad - if they're another religion, you're fine, or if you have an actual claim on the vassal's own lands. Otherwise, you've just assumed the role of their previous liege.
I can see why might want to remove the vassals form an RP perspective, but otherwise, you can placate them fairly easily - they weren't declared war on directly, so they won't have any negative modifiers towards you.
If you still really want to revoke all the old vassals, you can fabricate claims, spy on vassals, or simply kill them off - plenty of ways to deal with them, if you can wait. If you can't...just revoke them all, and deal with the opinion penalty (also, if you get imperial administration/ chinese imperial, you can revoke the duchy vassals for free).
I can see why might want to remove the vassals form an RP perspective, but otherwise, you can placate them fairly easily - they weren't declared war on directly, so they won't have any negative modifiers towards you.
If you still really want to revoke all the old vassals, you can fabricate claims, spy on vassals, or simply kill them off - plenty of ways to deal with them, if you can wait. If you can't...just revoke them all, and deal with the opinion penalty (also, if you get imperial administration/ chinese imperial, you can revoke the duchy vassals for free).