Veggie Grill, with two locations in Irvine is probably the best known vegetarian specialty restaurant, but almost any of the many Chinese restaurants at the intersection of Jeffrey & Walnut in Irvine will have extensive vegetarian selections and of course most Indian restaurants will have extensive vegetarian selections as well. Wheel of life in Irvine is a vegetarian Thai restaurant and almost any Thai restaurant will make your dish with tofu as the protein instead of chicken or beef.
Native Foods Cafe also has three locations in OC and is noted for their vegetarian foods.
There are a lot of good Vietnamese choices in Garden Grove/Westminster, look at Hoa Sen Vegetarian Restaurant, Bo De Tinh Tam Chay and Thuyen Vien.
Mas' Chinese Islamic Restaurant in Anaheim has a lot of vegetarian options too.
True Food Kitchen in Fashion Island isn't vegetarian, but has a lot of good options and some easy to modify options as well.
Lastly, Avanti Cafe on 17th in Costa Mesa has good vegetarian food.