Had a civ as vassal and was traversing their waters. They moved up an era and took on new civ. My vessel cannot move without causing a war.   Does not make sense to lose liege lord status instantaneously.  Mind you not clear why I want to be liege.  What does it give me?  conquered everything on the continent but could not eliminate the opponents so rather than just have them cede everything which they will not do, did the liege thing. What am I missing?

The first part sounds like a bug, but the Vassal system is also a bit unclear as well. Most games implement a power ratio, and if they outgrow that ratio they can break free, so possibly them moving up an era allowed that to happen?. 

You'd have a civ as your vassal as you get free access to all of their resources, and they have to follow you when in war. Plus you get free movement through their lands. I haven't noticed if there's a money element to it, though