When a person does or says something that is so undeniably characteristic of a vegetarian.
Examples:
Zach- "That'd be so great if I could get some extra spinach on that. I love spinach!" Person- "Man, that's So Vegetarian of you."
Person- "So you're vegetarian?" Zach- "Yeah, except for bacon." Person- *shakes head and thinks, "That's So Vegetarian."
Zach- "I woke up at 5 am today." Person- "It's Saturday..." Zach- "World Cup qualifying match. USA will win one day. Want a piece of my tofu?" Person- "Yup, should have known. So Vegetarian"
Zach- "I'll never change my Facebook profile picture. I love the plants in the background." Person- "Why?" Zach- "It was my prime...I don't want to disappoint people when they see a picture of me now" Person- "So Vegetarian."