So.... the catch is this... if you started the quest to eliminate the empire/rebels and one or more of them is low or have none fiefs and a couple of vassals with it, the bar will stuck at -100 till you reduces its number of vassals below the initial number...
The bad thing is, if you let that weakest member of empire/rebels for last, like i did, all the other vassals will be transfered to it when you destroyed the others...
So, fo exsmple, in this my walkthrough, i left the south empire for last...when i started the quest it had 3 vassals and 0 fiefs... after i ended the other empires, it received 10 more vassals...
So when i started to attack it for the end of quest, it was on -100 locked...
And in renained that way till i recruted 12 vassals from them... and then the empire falled and the quest finally ended...