I think it's important to hear why they're against it, then hopefully they let you say your piece, probably won't change a mind (but maybe!) but gaining respect for each other is the best you can hope for.
If it degrades, all I can say is that I don't push my thoughts, beliefs, and morals on others (we have a freedom here) and I don't respect anyone that does so. You don't see anti-vegan protestors!
I've always thought that Rinella's response to this vegan was top notch. I try to hit some of those points. I love the part where he points out that he probably knows more about deer than that person will ever know. Randy Newberg puts it in a great way too "I like to participate in nature".
It's always interesting, because there's different types to tailor your conversation: anti hunter, anti meat, anti anything fun...