It's characters in the Byzantine culture group that can do it: Greek, Armenian, Alan, Georgian. Surprisingly not Bulgarian, and really I think Irish should have it too since mutilation prevented someone from becoming High King. As for the vos, you can force your heir to take the vows if they're in prison. Toss him in jail and you're golden.Is blinding not exclusively a Byzantium feature? Btw I saw some people saying that you can always order your heir to take the vows, but that is not possible if he's heir to any titles.