I recently had the pleasure of organizing the food for my last honor society event, and I picked a pasta dish and a grilled vegetable dish. (It was the first time I've ever ordered exactly the right amount of food - I was pleased.) Neither dish had any meat in it, and I did this not to further my own goals as a tree-hugging vegetarian, but because of what happened at our fall event.
In the fall, I ordered meat taquitos and quesadillas for 30+ people (we had about 30 people in attendance), and ordered veggie quesadillas for an additional 10-15 people (because we had myself and a few other non-meat eaters.) By the time I finished serving the sangria, there was a half a plate of meat quesadillas left, but the veggie ones had all been scarfed up! I think I got one!
What happened is what always happened: damn carnivores! They take their meat, and then they see the veggie stuff, and think, "ooh, I want to try something of everything, I'll take some of this." So as a general rule, when ordering for a function, order equal amounts of the veggie and meat dishes, or more of the veggie if you know you have a lot of vegetarians.
For last night's event, I ordered pasta and grilled vegetables for 16 people (for a 30-40 person event), plus salads and bread, plus an order of chicken sate skewers for those who wanted meat. I highly recommend this approach when planning an event - rather than assuming everyone is a carnivore, assume everyone will eat the vegetarian foods, and then add in enough meat to satisfy your meat eaters. (If I had ordered a meat-based and a vegetarian pasta dish, we wouldn't have met our budget since there was a 10-person minimum for each dish.)
If you are not a vegetarian, and you are at an event where the vegetarian population has not been properly served (i.e. there is a small platter of side veggie things next to a lot of meat), please, remember the vegetarians and take only a small (or no) serving of the vegetarian dish. We appreciate it!