LET my daughter use a vibrator? Like she's going to ask permission first? You can't keep a girl from masturbating, and they're likely to improvise something as soon as they become interested. Let's just hope they have the sense to improvise something safe and don't hurt themselves in the process!
I think girls should have safe masturbatory aids as soon as they become interested in them. I started masturbating when I was 10, well before puberty and my first period, but I don't think I would have known what to do with a vibrator at that time. It took me a few years to realize it was OK and that I wasn't going to hurt myself or make myself sick. Somehow my parents neglected to mention that girls get wet when they get horny, and I thought I had made my body angry. If I hadn't improvised a vibrator out of a toy I had, I would have gotten into a whole lot of sexual trouble as a teen, I promise you. Safe and satisfying ways to masturbate kept me from finding boyfriends to experiment with until I was old enough to be smart about it!
I'm not a parent, but I've often thought about the best way to give your daughter a vibrator. I think my plan will be to give my daughter a sexual health box once she enters middle school. She can keep it in her closet or under her bed, and she doesn't need to use anything in there if she doesn't want to. I'll promise to stay out of her stuff and never snoop to see if she uses the stuff or not, so no need to be paranoid. I'll put some condoms in there (probably with a note about waiting to use them until she's dating someone she trusts completely), some books (I like "The Guide to Getting it On" right now, but I'm sure things will be different when I'm actually a parent), numbers or websites of places she can go for good information if she doesn't want to talk to me about it, and a brand new vibrator with some batteries. Right now, I'd pick a Layaspot, but I'm sure there will be better stuff out there by that point. Something quiet and non-threatening. Probably a package of Plan B, just in case. And I would let her know that if her toy ever breaks, I will happily get her a gift certificate to an online toy store and let her get what she wants without me snooping. (Of course, the box would not be the sole lesson of her sexual education.)