For starters, you have a responsibility to yourself. Your ethical duty is to parent in a way that matches your ethics and values. Sacrificing either, without explicitly explaining why you are doing so, doesn't do yourself or your daughter any good. So if you believe it's best to buy your daughter a vibrator, and you've squared this with yourself, it seems perfectly ethical. But there's more.
You clearly have an ethical obligation as a parent to your daughter. I would argue that your ethical duty as a parent extends beyond trying to minimize harm and includes a duty to create, as best you can, the conditions for growth. While buying your daughter a vibrator hardly seems like a necessary condition for growth, it can easily be understood to be one of many ways of helping her explore her sexuality in a safe environment. So thumbs up again.
On the other hand, your obligation to your daughter requires you to think not only of what you want for her, but how your actions may impact her negatively. For example, by doing something you know contradicts the values of another parent, are you putting her in a situation where she is colluding with you against that other parent? This may not be the case, but you're still the adult in this situation and you need to consider this before going forward. It isn't a reason not to buy your teenager a vibrator, but thinking on it might help you figure out what else you want to say to her beyond "don't forget to take the batteries out when it's not in use".
There is also the question of an ethical obligation to a co-parent. What, if any duty do you have to someone you are co-parenting with? If there has never been any relationship between you, your daughter and your ex, then this point is moot. But if there ever was a relationship then there always will be one. Whether you're in regular or sporadic contact, whether there is a romantic connection or not, there's still a relationship there. And where there is any relationship, regardless of depth or quality, there are ethical considerations.
I would suggest that the depth of the co-parenting agreement should in part guide how much or little you need to take their position into account. Do you regularly talk about parenting? Do you usually try to stick together on rules or values, or do you each present your own style? There isn't a right or wrong answer here, but it may be that you need to consider your ex's values more than you would like to. It doesn't mean your actions need to be guided by their values or wishes, just that you can articulate how your actions fit or don't fit, and why you chose to take them.
Buying a sex toy for your teenage daughter because you want her to feel empowered and have a good understanding of her own sexual response may be a great idea. If doing this contradicts a co-parents values, it doesn't mean you shouldn't do it. But it may mean that this is an opportunity to talk to your teenager about sexual values. It may present an opportunity to explore how there are very few sexual rights and wrongs, but there are infinite ways of experiencing a sexual situation or sexual decision. These lessons are arguably as important for her to learn as those that we all learn from our first vibrator.
