In this universe "The Justice League" are just comic book characters.
Supergirl of this universe "Universe 20XX" she is only a young girl that will never age again vampires has became a problem. Nyla was just a girl watching Supergirl on TV. One day She was on a car trip with her family when a drunk driver swerved and killed on impacted both her and her parents. The drunk driver was a scientist that was in a rut and he didn't mean to kill the family. So the scientist decided to take the dead family to his secret government facility to use them to create soldiers using alien blood to regenerate the bodies the best way it could. Supergirl was the only one from the crash to come back.
She ended up have super strength and flight used to constantly train to be the soldier they want she was only 7 so it was hard. The scientist who tried to help save all their lives was spited by Nyla she calls him "Doctor Doom" he entered her life , now she feels like her life was doomed. One day Nyla decided to watch Supergirl on TV then breaking news interrupted her show, Vampires are swarming the world. The alarms are going off in the facility the vampires are not being stopped. They bust into her room to bite and kill. She was not worried she was super strong now, but then she remembered her room is covered led and green stones that stops her power. The Vampire leader Bruce laughs then bites her, he then spits out the blood in discuss. Nyla starts convulse uncontrollably then her body just stops. The vampires move on thinking she died. Nyla has actually only been passed out for weeks her body was readjusting her dna with the vampires it was pain full, but now she is now a very powerful human alien science experiment with vampire abilities with the weakness.
When Nyla finally steps outside the sin was being blocked she got her strength from the sun, but now with vampire powers she doesn't need to rely on the sun. Doctor Doom was dead outside got dragged out the building no bites it was a slaughter mostly. Nyla sees a note book in his pocket takes it out and tries to read it and it was not just any notebook it was a journal he had information about the vampires apparently he was kid napped to build whatever it is to block the sun. He drank because he knew what he was forced to do could destroy humanity. He only got away because his government facility pulled a rescue mission which was bad for then. The vampires targeted the facility and did not care which city or person got in there way.
Nyla never thought about being a hero, but she figured she will start now to save her word. In the journal Doctor Doom had people and there addresses that could possibly help. There was a drawing of a machine that can apparently jump dimensions. She only thought that was possible in movies, but thanks to the book she decided she can use the machine to recruit people from other universes with power to help her stop the vampires not alone.
So this begins The adventure of Vampire Supergirl.