Pasta is always a good option. If you need a main dish, lasagna and manacotti, etc are so easy to make as vegitarian meals. Nobody misses the meat in those types of dishes, and smother anything in melted cheese and you can't go wrong! On that same note, pasta can also be a great side dish with pasta salads, or a Thai sesame noodle salad. These also go really well with a meat like skewered chicken/shrimp, etc on the side. That way you can also have something for the carnivores.
One of my friends threw a bruschetta party not long ago, and had a great spread with baskets of bruschetta toast, lots of toppings, and a huge salad. She made a sliced beef with horseradish, and lots of veggie toppings for the bread also. She had a traditional bruschetta tomato topping, a white bean spread, choppped liver, sauteed mushrooms, grilled veggies, and some great cheeses/italian meats. Made a great meal for a large crowd, and lots of it could be done ahead of time.