It seems to me this is less about veganism and more about this young woman's desire to estrange herself from you and your partner in favor of finding her own self expression and potentially re-connecting with her biological farther. It's likely that somewhere along the way she lost respect for you, most likely because of a perceived control imbalance. Now in an attempt to solidify herself concept she's reacting against the most grounded one she knows, yours.

This person is extremely frustrated at you for some reason. It will take all of your empathy to try to see what that is and remedy it. Even though she might be in the wrong on whatever issue, being right won't make her listen to you. You need to let this person wade into danger and then help them AFTER they get smacked up a bit by life. Buy her condoms and leave her alone, tell her she's in charge of her own actions and that she can totally come crying to you if and when they blow up in her face.

(Good luck, this one is hard. I fear the harder you cling the worse things will get)