My understanding of how witness votes work is that 100% of our VP is cast for each of the 30 witness votes we are availed. Witness vote VP does not deplete like normal VP.

Recently @edicted posted regarding how that enables Tron to vote 30 times with 75M Steem for 30 witnesses of it's choice, and how to make that not possible by causing 100% depletion of witness votes. Per his proposal, if Tron cast 20M votes for one witness, it would have 55M votes left. That eliminates it's ability to simply vote in 30 witnesses of it's choosing at it's sole option.

I also posted regarding the exact same idea that day, as I had read a comment from @bryan-imhoff explaining the idea. Apparently it's not a new idea, except to me.

I strongly recommend Steem protect it's witnesses via this mechanism, which decentralizes control of the witnesses to the degree it can be done on a DPoS platform.

Also, Vote for Witnesses!