I don't think a conversation at a birthday of a friend qualifies as "public" in the traditional sense.

So I might bring that up with people, as the topic arises, and advocate for it (i.e., tell them why I think not eating meat is better).

I think that's seldom the most straightforward way for changing people through personal conversation. It makes much more sense to ask a lot of questions and target your communication at other person,

Status also matters. Sometimes doing something weird lower your status other time it raises it. It always makes sense to look at the individual situation.