"Especially if you add the innocent tweak that you can´t swear vasselage to another kingdom but yours after you took a minor faction sacked town. The PC should be forced to autmatically declare souverignity after he conquers such a town and doesn´t hold at least a village."
@Noosers
I think such a thing is already in place, kinda (not fully as you suggested).
Example: Laria was attacked and sacked by the Noldor, the Empire took advantage of this and took the town. Save game!
I wanted to see if it was possible to get some nice holdings so I took Laria from the Empire and ran to Ulric to join Sarleon. Ulric refused to accept my desire to join Sarleon telling me he would not have any lord with stolen territories.
Sadly I do not remember if I ran to another lord to check if I could join them.
In my save game though, the only places that has been sacked is Laria and Cez. Marleons have been tried, but they left quickly. It is so strange seeing D'shar holding Cez...
(Since there is so much Empire land between them and the nearest D'shar village.) (Only thing that would have been sweeter with the sacking of cities were if they took them for their own. (muhaha)