I know how it works in RMXP.
i.e. Enemy has A rating for fire. If you use a fire attack, you will do 200% damage. The damage ratings are 200%, 150%, 100%, 50%, 0%, -100% for A, B, C, D, E, F respectively. Same goes for actors. If an actor has fire rating A, he will take 200% damage from fire attacks.
That's for XP, but I doubt that it's different for VX.