Of course the daughter should cook her own vegan food and clean up after herself. But I do agree that it's OP's responsibility as the parent to let her daughter know that she has the option to cook separate meals for herself, and what responsibilities come with that, and what budget she would have per meal or per week. OP only said that her daughter never "offered" this, but of course she wouldn't. When I was a teenager, cooking separate meals for myself simply wasn't an option (although I did learn to help cook for the family at a young age), which is why I didn't go vegetarian until I was older and independent. But if cooking for herself is an option as long as she stays within a certain budget and cleans up after herself, then OP should let her know! Of course a teenager might not "offer" to do these things or know what a reasonable budget should be, but that doesn't mean they wouldn't learn if given the opportunity. It's the parents' responsibility to say "Hey, if you want to go vegan, you're going to need to learn how to cook within this budget, and you're going to need to keep the kitchen clean".

OP is NTA. But the daughter is 15. She's learning. And she might be a little overenthusiastic about her new passion (veganism) without having the necessary skills to fulfill those values on her own. I think with a tiny bit of guidance, she could quickly learn to cook nutritious & delicious food on a budget. Why not give her a chance? And if it doesn't work out, then it doesn't work out, but I think it would be worthwhile for OP to at least give her daughter that opportunity.