LuViNLyFe - the picture you painted about the parents made me smile (""no meat either huh?" with a scrunched face"). I can totally see it. People are so funny sometimes, especially adults. Kids tend to be way more accepting and just let things slide w/o making a big deal of it.
As far as how to respond to something so specific and direct..... I'd probably give a slight smile, nod, and say "Yep." :-) Seriously. I don't see any need to launch into a long explanation for that parent who is judging you or looking down on you for your food choices. They probably don't care to hear it. If you get the feeling that the person is genuinely interested in your no-dairy, no-meat plan, I'd talk in more detail with that person. Tell them that you've found that this kind of diet is just overall healthier, so that's why you're doing it. Maybe say you came across this awesome book, "The Kind Diet" and it really opened your eyes to a lot of this stuff. (I've said that to some people who've asked me about it).
It just depends on the attitude the person has about it and the feeling you get from them. Sometimes when you go into a long explanation for your behavior it comes across like you're defending yourself. But you don't have to defend anything to those other parents. That's why I'd just answer with a simple, "Yep, that's right," and go along my way.