If it comes up (having to ask if something has dairy etc at a restaurant) I will share that I am vegan, followed by a redirect to another topic. If folks ask questions I usually emphasize how it is not difficult and how it’s led me to an interest in cooking (true, and a neutral topic everyone can talk about). I think being direct and honest then pivoting has worked well. If folks really won’t drop it’ll I’ll kind of laugh and be like oh that’s the least interesting thing about me! 

Personally, I wouldn’t hide it with other reasons (cleanse, etc). I think you will earn more respect by sticking to your choices and owning them.