I'd second Charlie Trotter's Vegetable Tasting if it is within your budget for clients. My family includes two vegans and his kitchen had no trouble providing a spectacular purely vegetarian meal on no notice.
In the city, I like Karyn's on Green
for a purely vegetarian option. It's tasty, bright, modern, close to downtown and 100% vegan - so no
animal products which is more restrictive than your customers' demands but certainly offers a 100% safe option. From Karyn's website: An Earth-friendly vegan approach to contemporary dining. Our diverse menu features classic American dishes reinterpreted with innovative presentations but without the meat, fish, chicken or dairy ...
Further out of the city, but certainly worth the drive, in Glen Ellyn is My Kha's
restaurant - a below-the-radar jewel with lots of options for vegetarians. The menu is built on a very non-vegetarian Vietnamese cuisine but also offers vegetarian renditions of many dishes, for example, Waterfall Tofu or Shaking Tofu. Then, the owner and chef, My Kha, will happily (better with advance notice) customize any dish on request or create special dishes. Her home-made tofu, fresh fruits and vegetables, mochi, tapiocas, sorbets and sauces are as flavorful, delicious and healthy as you could hope for in a restaurant setting. My Kha's has been our family's go-to for over two decades. (She also serves up a lunch buffet that is offers a vegetarian delight with half-a-dozen options that change every day.)
Karyn's on Greenhttp://www.karynsongreen.com/
130 South Green Street
476 Forest Ave
Glen Ellyn, IL 60137