I think the VOL is short for VOLUME. It means the tyre is quieter than a non-vol tyre. There's usually a strip of foam around the inside of the tyre to absorb noise. You can get identically branded tyres, one with VOL, one without. The VOL will often cost a bit more
When I needed a new tyre the fitting place had the same tyre but it was Mercedes spec and didn't have VOL. As my car is leased I though i'd better stick to the originals and ordered the correct one.
I don't see how there could be any difference in performance unless the VOL one is heavier because of the foam?!? or if this is taken into account and has less rubber?