I actually want a vouluntary vassal to have the ability to break away. They may have become a vassal in the past because they didn't really have a choice. But as time goes on they may get very powerful, and at some point announce to the world that they are 100% sovereign and a full fleged civ. The vassal basically tells the civ that "owns" them, "take it for what it is. If you don't like it invade us." This is realistic. A vassal that becomes powerful will not want to remain a vassal forever.
And its something to think about and plan for. If a civ becomes your vassal you can't count on it forever. One day they may become your enemy and maybe even make you their vassal.
And its something to think about and plan for. If a civ becomes your vassal you can't count on it forever. One day they may become your enemy and maybe even make you their vassal.