If one considers 50 MW (net) wind or solar farm a 1.0 MW capacity of the VRB would store and discharge 2 % of net production continually.
When one considers the total cost to put in the 50 MW (net) installation, the cost of the completed VRB (my guestimate) could be over (5x - 8x) the $600,000 cost of material alone. In effect the cost as a percentage of a total 50 MW (net) Plant is not high.
Relative to EV charging times and distances traveled for the Vanadium, Lithium, Phosphate Battery, check these news releases.