Vanadium redox flow batteries have energy densities of ~10–20 Wh/kg. But for grid storage I think even sodium-ion batteries would be a better candidate. Those have energy densities of ~150 Wh/kg. Vanadium is mostly produced as a byproduct of mining magnetite iron ore deposits. So, sodium-ion is less dependent on the output of handful of mines globally.

Having a hydrogen economy is overall pretty inefficient. I think it's largely a dead end. We do not need multi-year energy storage. Seasonal storage is needed at most. Lithium-ion batteries have self-discharge rates of around 2% per month. Even if the batteries lose ~10% over a few months that's an acceptable loss and is superior to the inefficiencies inherent in hydrogen production via electrolysis and the subsequent use of stored hydrogen via fuel-cells — which require platinum-group metals.