I think the vassal system where you earned all of the vassal's votes was more of a puppet state system, which is not necessarily a vassal. Also, a vassal in VP basically meant you earned a glorified city-state ally, which is not really how a vassal worked.
A vassal system is basically a mutual agreement between 2 nations, one of which is obviously superior to another. Since a vassal in VP can only be earned through war, part of the mutual agreement should include the defensive pact and open borders which currently happens. The tax system used currently is more of an imposition than agreement, and instead there should be some tradeable asset(s) the vassal can agree to provide for your protection, your mercy, and your strength. Gold could be a part of that agreement, certain types of votes, or even stuff not in the diplomatic game (if anyone wanted to program it) such as great works, great people, or great people points, free conversion of all cities to your religion, or whatever. The key is it is a mutual agreement and the agreement is not forever. It has a duration, perhaps longer than normal trade deals, definitely longer than a truce, but still finite.
I am glad that vassalage isn't exactly an automatic puppet state now in the new beta.