Does Cinderace VMAX's "Counter" attack do extra damage if Cinderace VMAX goes to the Bench and then returns to the Active Slot?
It doesn't matter where Cinderace VMAX was when it was damaged during your opponent's last turn, nor whether it has switched in or out of the Active Slot. If it was damaged by an attack during your opponent's last turn Counter will do that much more damage. This is not an effect to be removed when the Pokemon goes to the bench. Counter is performing a game state check asking if something happened; since it did, it uses that game state information to calculate how much damage Counter will do.
Counter