You could try to set up some sort of siphon system. If you threaded a piece of aquarium tubing down under the marbles you could then use it to suck all the water out. If the tube was long enough and the vase high enough up, you could use gravity to do most of the work, once the siphon was started. Sorta like the roman fountains. This would probably be rather obvious though, as you'd have to have a coil of tubing attached to the plant.

If you could get a glass tube that could reach from the bottom of the vase to just a bit above the marbles, and aquarium tubing that fit over it snuggly, you could do the same thing but detatch the plastic tube between water changes. That would be prettier, but I don't know where you'd need to look for a glass tube. :-/