Vampires comprehend the law of blood which they have exclusive access to while the Wicca with the eye of weakness have access to a false law.
The Supreme Alliance was able to defeat this race despite their ability to identify weaknesses because Supreme beasts are most aware of their own weaknesses and they have each other and their numerous divine abilities to make up for it.
Unfortunately, the Supreme Alliance couldn't make full use of the divine ability they gained from the Wicca race after gaining victory over them. Their failure is similar to their failure to crack the ability of healing and resurrection of Vampires. They can't comprehend the false law of weakness without the permission of its creator but they would rather not gain that permission because it would mean giving up their existence to that creator.
