It might be in a completely original universe fanfiction where the author creates their own rules and lore for Dawn's transformation into a vampire. This could lead to unique storylines and unexpected outcomes.
Maybe it's in a Buffy the Vampire Slayer crossover fanfiction where the rules of vampirism are expanded to include Dawn. There could be all sorts of interesting plot twists and character developments.
Well, it could be in a Twilight-inspired fanfic where different characters have their supernatural transformations.