In lead acid cells the energy is stored in the plates of the cells. So the cells are rated primarily by the amount of energy they hold, not just by how fast they can absorb or release the energy. Also, that energy absorption/release rate is proportional to the storage capacity for any given type of construction.
In vanadium redox cells the energy is stored in the oxidation state of the vanadium solutions, which are stored in tanks and pumped into and out of the cells. So the amount of energy the system can hold depends solely on the size of the tanks. The only rating that is significant for the cells (and pumping system) themselves is how fast they can absorb or release the energy.
Vanadium redox cells are essentially a type of fuel cell. The cell is the engine and the storage is in the "fuel tank(s)".