What's your objective? Probably you make fun of them because of your own emotional needs. Yes, you'd like them to change, but if you were consciously coming up with a method to engender change, you probably wouldn't choose mocking and alienation. Teasing is more likely a way of protecting yourself and your feelings, and trying to carve out some social space for them to exist in. Which is valid.
The single best thing any vegan can do to make more vegans is to be a visible positive example.