The quantity of VP is not stable. Every vote I cast at 25% VP changes the value of every other vote I cast, because my VP is depleted as I vote, and recharges when I don't. It's not possible for VP to remain the same when it is depleted to vote.
It seems you envision the VP slider eliminated. Not sure I like the idea of only being able to vote at a level set at the factory. I settled on 25% because that best fits how I use Steem, allowing my VP to recharge nominally to keep my votes relatively equally valuable.