It requires some micro, but the game should stop when you receive a vice-duchy, just grant it back.
The reason it helps you is that each that the vice-duke or vice-roy dies, the title goes back to you and you grant it back, you get an opinion boost and it will help you keep the realm stable. For some reasons, people tend to not like vice-duchies and kingdom but I'm telling you : KEEP THEM, IT WILL HELP YOU.
When you start as Byzantium, a lot of laws are enacted for you, such as vice-duchies and vice-royalties.
