Being that the chocolate has already been paid for, the damage is already done.
Some vegans are vegans because of personal health reasons. So those vegans wouldn't eat chocolate anyway.
Some ethical vegans would give it away to some nonvegans with the idea that they would buy less nonvegan chocolate in the near future having eaten your gift. Therefore creating some good. Plus you would be healthier. And maybe also incur some goodwill.
For me, it would be a really hard choice cause I only buy chocolate that is ethically sourced, too. But I have many nonvegan chocolate loving friends. And I could easily re-gift it.
But I love chocolate. And if there was some THC involved it probably wouldn't make it all the way home.
If you do decide to eat your chocolate I can promise you that the Vegan Police will not be breaking down your front door at six o’clock in the morning because of it.
Your real problem has nothing to do with chocolate and everything to do with relationships.
Once Auntie Margaret discovers that you are vegan but eat her milk chocolate then it will keep coming. And there might well be a leather belt for your birthday and a sheep skin coat for Christmas. Just because people know you are vegan does not mean that they really understand. Far from it. “Oh, but you do still eat eggs don’t you.”
Personally I would say something like this, “Thank you for the present. But because I am now vegan I no longer eat any dairy products at all. So I decided to give the chocolate away so as not to waste it. And what better person to give it to than you. Please do enjoy it. I do love you to bits.”
Not only will Auntie Margaret get the message without being upset but she will spread the word through the grapevine far more effectively than your ever could.