I am wary of encouraging passive aggressiveness, because I do think a calm, thought-out confrontation expressing your honest feelings is always THE best way. But for difficult situations, when I'm being harassed by someone I don't really know- I've learned to either one-up them, or basically ignore them in a way to create a very uncomfortable an awkward silence (which usually leads other people who are around to stop and think "wow, what an assh**e!").
I think I've developed this from many many years in the customer service industry where, you CANNOT confront someone, even if they are being extremely rude and vulgar (because you either won't get a tip or will be fired). Many times I had to approach a table full of a bunch of college aged guys who couldn't wait to make me (a girl) squirm in front of his friends.
So I became pretty quick at coming back with a better response to shut them up, or blatantly ignore them and try and make THEM feel uncomfortable. By blatant I don't mean pretending you didn't hear them, I mean make it really obvious that you heard them, and you are ignoring them.
On Saturday I attended a wedding and it was great because the bride made sure that I had a vegan meal. I was at the same table as a young, cocky, and immature guy who seemed to constantly need to be the center of attention. He made some comments, at which point I shot things right back at him (meaning instead of him get a laugh, I got a laugh). Then finally he made some idiot remark about "Don't worry, the rolls are only made of pine sap"
At this point everyone looked at me, and I made a face (I just look really really bored), turned my head to the guy next to me and said "WOW THAT WAS FASCINATING, EH?" Then I just went on with my business. That shut him up.
Never ever let people make you think it's you. It is utterly ridiculous that anyone question things that are YOUR business. Everytime he says stuff think "THIS is an adult?!!"
Believe me, it will get more and more dull, and you can work on your "bored face" this way
Not sure if I explained that well, or if it works for you, but it always seems to work for me.
Consider him practice for all the dummy comments you'll be getting as a veggie
stupid comment=bored face-->then something like "Geez, being a vegetarian is really annoying, and I ain't talkin' about the giving-up-meat part"