Vampire's initial strength depends largely on the power of who turned them. While both Akasha and Eris were born vampires, which means they inherited that trait from their parents, Lucia and Zia were turned into vampires by another person.
Arad is in no way close in power to someone like Akame, the red-eyed goddess of undeath, Akasha's mother. That means that whatever start Akasha got from her mother, that's the best she'll ever have.
But that story is different for Eris, Zia, and Lucia. Now that Arad has both Akasha and Vlad's power, he probably could do something to help.
For vampires, there are two methods of getting power besides hunting people and drinking their blood. The easiest one is serving another stronger vampire and getting your curse re-applied by them, while the other one is dangerous, absorbing the blood of another vampire.
