Ok so I tried it before and after adding vinegar and it just tasted like bitter nasty root beer both before and after.
The root beer overpowers everything and the kvass is just gross mixed with other stuff.
I have therefore made evil juice™ II.
Its everything you could want from evil juice™: evil green colour, unplacable unnameable Eldrich taste, pleasing evil sourness and I can really feel the evil >:) coursing through my veins ψ(｀∇´)ψ