If you're looking for restaurants that are exclusively vegetarian, then it's slim pickin's, we regret. Adar Pizza is a vegetarian and kosher pizzeria that has a good reputation (we have not tried it), but aside from that, the only other options are cafés of one shape or form.
However, that's not to say that Las Vegas doesn't cater to vegetarians and vegans, provided that you don't object to having carnivores among your fellow diners. Depending on your tastes, there are a number of choices.
We happen to love Indian food, and Las Vegas has become increasingly blessed with good Indian cuisine, which affords the vegetarian (and often vegan) diner all manner of interesting dishes to choose from. Himalayan Cuisine is a cosy modern restaurant that's reasonably priced and has impressed us the couple times we've eaten there; Bollywood is also good, as is Shalimar (an old favorite that closed, then reopened at a new location as more of a café than restaurant, but still serving good food). If you're looking for something fancy, try Origin India, which is excellent and definitely upscale in ambiance. There's a great coupon for 25% off your entire bill, including alcohol, in the 2010 MRB, too.
Asian food generally offers plenty of options for vegetarians and we can particularly recommend Lotus of Siam (casual décor-wise, but consistently considered among the top Thai restaurants in North America) and Komol, located in the same plaza, has definitely seen better days in the looks department, but is excellent and serves one of the largest vegetarian and vegan menus of any restaurant in Las Vegas. China Mama is a new discovery of ours that we were turned on to by the authors of our upcoming Eating Las Vegas guide and we've had some interesting dining experiences there. We're not experts so the menu can be hit and miss (this is not P.F. Chang's), but we've found some awesome dishes through trial and error.
Sake House Ichiza is another LVA favorite -- again, it's a casual, bustling spot with daily specials pinned all over the walls, and is great for late-night dining. The food's excellent and veggie friends of ours have no trouble finding interesting and tasty dishes to try (we can particularly recommend the broiled tomatoes & eggplant with mozzarella, and the four different kinds of mushrooms). We'd also highly recommend Raku, which was a James Beard finalist for Best New Restaurant last year (try the potato with corn and the fried hot green chilli pepper appetizers).
Otherwise, there are lots of good Mediterranean restaurants (we'd particularly recommend Ali Baba, which Lebanaese poker pro Freddie Deeb turned us onto) and, of course, Italian joints that are vegetarian-friendly. Or you can try something more exotic and check out one of the growing number of Ethiopian restaurants that Las Vegas is home to.
For details of all these, plus many more suggestions, visit our Restaurant listings in "Buffets & Dining" and scroll down to "Vegetarian-friendly," or else click the "Veggie/Vegan button for those options.
One final thought is that buffets generally offer a good array of vegetarian dishes, salad bars, and so on. You can find one to fit your budget and inclination -- if money's no object, then Bellagio and Wynn in particular both offer high-end fare in elegant surroundings (particularly the Wynn).
Bon appétit!
