The Vassalage section allows you to register the relations of fealty between factions.
Click on the cross icon to remove a relation from the table. This means that the factions will no longer be liege and vassal, and the vassal will no longer contribute to the income of its liege.
A popup window opens when you click on the “+” icon. If the selected faction is the Liege, simply select the vassal faction and the Liege will automatically be filled in with the selected faction, and vice versa. Select the type of vassalage and click on the “Add” button to create that new relation between the two factions. Click anywhere outside of the popup window to cancel the change.