I don't think there's anything wrong with it per se, but I wouldn't buy her one--for a few reasons.
For one, if her mother finds it and gets upset about it, it could come out that you bought it for her which could cause a big shit storm. You can encourage sexual exploration within the safe bounds of masturbation, and being that sex-positive voice in her life could be a wonderful thing for her, but that doesn't mean she has to do it with a vibrator. There's a reason why you have to affirm that you're 18 when you buy from a shop like EF, and why you have to show your ID when you go into a shop--people can get into serious trouble for supplying sexual material to minors. I wouldn't risk it.
I also think that exploring yourself through masturbation is something that happens in stages. Vibrators and sex toys are nice, but perhaps it would be more helpful for her to explore her body on her own, find out what feels good to through human touch first, and then think about the fancy toys later. Maybe she already has started this journey, but maybe not. I started masturbating when I was 10, and I had no idea what a vibrator was (I remember my friends laughing at me when I didn't know what a dildo was in the 5th grade), so it's not like she has no options for exploring her sexuality.
If you were her mother, I'd be more inclined to say sure, go for it. But you aren't, and that can be risky. You can never be too careful when dealing with minors and sexual matters; and tread even more lightly when it comes to your underage family members. What you see as harmless or even positive, her mother could see as crossing the line.