What threat do my prisoners pose?

So, after a small revolt, I imprisoned a few of my vassals. This made an excellent opportunity to restructure my realm. Those who had a different culture had their lands repossessed immediately. I do however have two remaining prisoners, whom I don't know what to do with.

One is a duke and double count. I could strip him of his primary title for free but not the counties. I'll let him rot until he dies, methinks.

The last one is a mere count. I was thinking of removing his title, but then I started thinking ... "Well, at least I know where I have my dear vassal"

While he is imprisoned, he can't rebel or plot against me. So that's neat! But what are the down sides of keeping your vassals locked up? Will his heir come back to haunt me?

Solution 1:

His close relations will have a significant negative opinion penalty of you, so if his heir is his child or another close relation, yes, imprisoning them has already poisoned your relationship with his heir. The "imprisoned my parent/close relation/child" penalty will expire eventually after releasing a prisoner, but it won't expire while you have them captive and they still live.

Your vassals are also unable to do useful things as your delegate within the realm: they can't declare wars, or pacify their vassals as effectively (due to the diplomacy penalty from being in prison). That may or may not matter to you, or may actually be an advantage politically, depending on the exact state of affairs in your realm.

Personally, I tend to strip such counts of their title, assuming that won't cause political trouble with their relatives. A county title I have free to gift to someone loyal is much more valuable than having a vassal where I can keep an eye on them.