Vat is an extra store of electrons as run out reserve. The product form is such that it can be easily swapped and hung on the rear of the vehicle. These vehicle saddle bags could then give a replenishing charge to the charging ports if required.
If liquid batteries are used, a mesmerizing jiggle would
be afforded to other drivers and pedestrians. Built in, special overflow pockets could act as spare capacity in bouncy minor mishaps.
If SuperCaps were used, a minor accident would make them unreusable. Although, because of new engineering design, any crush would give give a higher output due to reduced charge separation aiding getting to the shoulder at lightening speed to swap insurances.
Waiting for charging will be so passé, as you can charge on the fly and quickly stop at the swappa-shop for quick Vat pack.
So don't just add junk to the trunk, add power.