Chicago is really a great city for a vegan/vegetarian because I've found that, if places don't readily have something vegan on the menu, most chefs find it to be a delightful challenge to make something special for me. I've even been to Moto and they've made a 10 course vegan meal for me!
But, if you'd rather go somewhere that won't require a phone call ahead of time, here's some of my favorite places to go downtown:
1. Le Colonial
They have a separate vegan menu (which most of it's on the regular menu, but they do have a few special and/or modified items). It's upscale Vietnamese food and I've never had anything there I didn't like. Also, the cocktails are fantastic, and I love the 1930's style decor.
2. Gilt Bar
Admittedly, a small offering of vegan faire, but they do it right. The roasted garlic and balsamic on toast is to die for, as is the cauliflower. The decor is chic and rustic, and sometimes they have live music (which manages to never be too loud).
3. Karyn's on Green
All vegan, all amazing. Again, try the "cheese" plate and the "crab" cakes. Unbelievable!
4. Sunda
Japanese food is usually pretty easy (provided you make sure that there's no fish in the miso soup or the seaweed salad). I found that this place had lots of really unique, vegetarian offerings. And because it's kind of a tapas style place, you can easily fill up on tofu, seaweed salad, edamame and veggie sushi. Oh, and they have a great selection of sakes!
Hope that helps, fellow rabbit food eater