I'm not sure exactly what your asking but this is how I understand it to work. Hit your target as if you have defense piercing but you do not attucaly pierce their defense. Amplify that hit by target (armor defense + vit defense)/100. Even though this is effectively multiplied onto that hit like a turtle general multiplies your hit it doesn't actually multiply the hit. It adds attack to your attack until it comes to what that multiplier would come out to.