Play as a Germanic nation, become emperor, revoke privilegia. Form Germany. The previously free vassals now give you -50 or something diplo mana per month. Can't annex them all due loss of opinion at annexing, and after becoming Germany, due to lack of diplo points. Can't release them due to lack of prestige. Could try going to war and losing, but odds are the winner wants something else than releasing vassals, and would take many wars due to warscore limit. How to deal with costly vassal swarm?