I haven't been to any events done by these caterers, but here are four I met with at the Vegetarian Food Festival on Sunday:
espritevents.com - their sushi is delicious, creative and gorgeous.
regalvegan.com - great banh mi sandwich
veritecatering.com - I've only had their sweetshttp://www.ieatgrass.com/
- Ayinde's mac and yease is pretty much famous from the West Cafe waffle brunch, but he also does a hell of a "tuna" sandwich