The Ninja Vampires are one-time villains in the Phineas and Ferb series, only appearing in the episode "Nerds of a Feather". They serve as the secondary antagonists of a TV pilot for a new television series called "Doof 'n' Puss", based on a story pitch written by Heinz Doofenshmirtz to television producer Jeff McGarland during the Tri-State Area's annual sci-fi and fantasy convention.
According to the "Doof 'n' Puss" pitch written by Doofenshmirtz, the Ninja Vampires' leader the Shape Shifter is plotting to take over Danville. To do so, he captures Danville Mayor Roger Doofenshmirtz and impersonates him in order to secretly write up a law that legalizes Ninja Vampires.
Due to the Shape Shifter's obnoxious and cruel behavior while posing as Roger (such as transforming into a werewolf during a ribbon cutting ceremony), Doofenshmirtz (who is written as a rogue-trillionaire inventor-extreme fighting champion in the story) and Perry the Platypus are assigned to spy on the Mayor. To do so, they disguise themselves as stage magicians in hopes of gaining an act for the Mayor. However, the Shape Shifter learns of the duo's true identities after getting a picture of them buying the disguises from a store, so he sets 20 of his Ninja Vampires to capture them and hold them prisoner inside a storage closet where the real Mayor is being held at.
Perry eventually manages to free himself, Doof and Roger with scissors, and they all manage to expose the Shape Shifter as a fake to the public, forcing the angry Shape Shifter to transform into a giant monster to attack the heroes. It is unknown what happened to the Ninja Vampires afterwards as the story was cut off by McGarland, who decides to pitch the story as "The Platypus and His Girlfriend" when Doofenshmirtz refuses to give Perry a girlfriend in the story.