There could be. The Star Wars fandom is vast and full of creative minds. Writers might want to delve into the dark side of the Force and how Ahsoka's relationship with Darth Vader could develop in such a scenario. Maybe she is forced to work for him under some kind of mind control or perhaps she has her own motives for being in his service that are gradually revealed as the story progresses.
Sure, it's possible. In fanfic, many different 'what - if' scenarios are explored. Ahsoka working for Darth Vader would be a very dark and interesting concept. It could show her struggle between her Jedi teachings and the influence of the Sith. And it might also explore how Vader himself views her, whether he sees her as a tool, a reminder of his past, or something else entirely.
Yes, there are likely some fanfics with this concept. Fanfic writers often explore unique and alternative storylines. They might be interested in exploring what would happen if Ahsoka, who was Anakin Skywalker's apprentice, ended up in a situation where she worked for Darth Vader (Anakin's Sith persona). It could involve complex character dynamics and a re - imagining of the Star Wars universe.