to further elaborate..... the two catagorys is inheritance or revokation.
inheritance requires dynastic extinction, while revokation requires you to eat tyranny or get a lawful reason to revoke, the latter of which includes rebellion, which can be from natrual factioning or failed lawful imprisonment , event basaed traitor flagging from stuff like intrigue focus, and generating claims from councilers or events, or Catholic Popes.
for a Roman Emperor, the best way to avoid a lot of trouble is to use the CB on a Multiduke or a Duke with multiple countys that are outside his Duchy, you dont vassalise the duke under those circumstances and gain the countys/duchy personaly and can hand them out any way you want.
unless your fortunate enough that the duke you vassalised has a small dynasty. wiping it out can be costly, plots over time will accumulate modifers like known murderer and other opinion penaltys when someone inevitably gets drunk and reveals your current plot, with the amount of negatives you will accumilate on average its almost more easy to eat the tyranny penaltys and let it tick down, especialy if you have an empowered council and your major vassals wont faction on you.
intrigue focus is RNG and you could be waiting decades for an event to give you a revoke cause on whovever your spying on, or have it happen in a week, its a dice toss. you an also get imprison justifications to force rebellion.
If your Catholic you can vassalise the Pope and pump him for claims, probably the most efficent way to turn fedual vassals into viceroyaltys. other main source of claims is the counciler and duchy claims have a long Mean Time to fire.
Orthodox excommunications can help you force rebellions, but the system hasnt been touched since vanilla so the emperor has no way to excommunicate anyone outside Constantinoples patriarchite, there really needs to be more diplomatic options where a ruler can interact with other patriarchs for these things. its only useful in the early byzantine game for vassal management and pruning heirs (excommunicate, imprison, and blind to disinherit heirs you dont like)
So, unless you EZ mode it as a Catholic, converting new aquisitions to viceroyaltys will be a Slog even if you apply multiple methods. the worst its going to do is slow you down, if you got a good portion of your Roman territory from holy wars you can take the time and pace yourself eating through Reconquest territorys.
CM startdate has the advantage of the most time and Christianity as a religion is its smallest, you can open a conquest front in Iberia from the start since the majority of it is muslim, if you back off and let them eat Asturias and Aquitine its better for you. If Germanics eat into Brittinia or Germany it also makes things more easy, likewise if you stay Orthodox and rulers stay catholic after Mending.
