Is there any way to directly feed on / drain a prisoner without having them in your herd first? Either a mortal or a vamp? Because if there is I can't find it.
And the wiki specifically says "A prisoner can be directly diablerized", except they can't. Hovering over the option in-game lists it being blocked by the fact that "so-and-so is imprisoned by you".

It seems to me your average vamp should be able to walk right up to anyone they have in chains in a dungeon and drain them dry without putting into the pen with the rest of the cattle if they so desired.