While I understand your desire to get it back, most mods don't have vin numbers, at least not yet. Keep in mind that if your "friend" decides to involve the authorities, well, you have no proof she actually did it. If it was a legitimate purchase on her part (although it sounds like maybe not) you may be the one winding up with more trouble, not her. I also don't recommend "stealing stuff back." As "stealing back" is something the authorities also don't tend to take kindly to. I would maybe confront your friend, ask her to be honest, and that she will lose your friendship over a $50 mod, and then stay away from her if she doesn't respond. Sounds like you are pretty bent out of shape, which I totally understand, but also, understand that these types of situations can swing out of control rather quickly, and it may not be worth it to pursue matters.
I would consider just cutting off contact with this person, and moving on.
I hope you get a resolution, but I mainly hope you get a GOOD resolution, and that may mean letting the situation go.
Welcome to ECF.
Anna