If being a vegan is an important part of your identity, you must tell her. If you can’t tell her about an important part of your identity, that’s a problem, and it has to do with trust, not with veganism. If it’s not really all that important to you, then telling her shouldn’t be a problem. Right? (But wait… don’t you ever eat together? why wouldn’t she already know? Have you actively been hiding it?) As to the reply, “Don’t be a vegan”: Hm. Well. I’m a post-it-arian, so far be it from me to criticize or attempt to manage someone else’s dietary practices.