Maybe one of the reasons why they interrogate is because you're trying hard to keep it secret and dislike explaining yourself? Some people pick up on that stuff subconsciously. If you were open about it, it would become less of a big deal to the people around you. The more you hate explaining yourself the more people feel like probing, either because they're curious or just malicious.
If someone asks or needs to know because they're cooking for me, I just tell them straight up. If they're asking because they're curious I answer any questions as long as they're being respectful and not taking the piss. When they take the piss I just tell them, "it's no big deal" and act like it's no big deal myself. Don't say anything like "what's your problem?" or "why can't you just leave me alone?" or "stop being a dick" because it just makes it all the more tempting for them to keep at you, you may not realise it but it's an invitation to keep going.
With a person like that I try not to invite further conversation about it, I either ignore them or shut the conversation down. And if I have the choice I don't have anything to do with them again, especially if they're a malicious wanker.
Never try to put them in their place with questions or statements that imply they're mean, ignorant, stupid or an @rsehole, because all questions invite an answer and you want the person to shut up, you don't want their ego to feel the need to defend itself. Make a closing statement that they can't come back on or add to and then change the subject, or just ignore them and change the subject, whatever is socially acceptable at the time. Sometimes leaving the situation makes the bullying worse. Unfortunately when you look like an easy target, you become more tempting to torment. Ego's love easy satisfaction. Ultimately this all comes down to you being secure in yourself, secure that you can handle anything thrown your way.
If they're a friend and curious and a few jokes come out but they're a genuine friendly person, not malicious (I trust my instincts here) then I just laugh with them and let the joke breeze over, then go on with what I was saying or change the subject. No big deal.
The secret is being secure in yourself and don't make out like
being vegan ( or having to explain yourself or being interrogated about it) is a big deal.
Good luck
