How about these two niches:

1. non-vegans eating with vegans at a vegan restaurant, where eating there wasn't their choice (they were craving a burger), and so, being forced to order off this menu, will choose the most burger-like thing on the menu.

2. non-vegans eating with vegans at a non-vegan restaurant, where for whatever reason they feel the need to impress / not-offend the vegan by eating vegan food as well. (Think "first date" or "client meeting.")