I'll 3rd Lotus of Siam, except you'll probably have to make reservations. They've gotten a lot of press lately (as well as winning an Epicurean Award) and it has been nearly impossible to just walk in on weekends for dinner.
Also, check out Tofu Hut, Sam Woo and Noodle Palace in Chinatown (on Spring Mountain, a very quick cab ride from the Strip, and my faves). Actually, anything in Chinatown fits your criteria.
Oh, and Lotus of Siam is in Commercial Center, across from The World Famous Green Door *snicker*.
Be sure to check out any magazines in your hotel room, they are sure to have coupons for various restaurants.