There are few special restaurants for vegetarian in Xi'an. You can ask the waitors to suggest some food which do not contain any ingrediences made of animals and birds. There is a buddism restaurant for buddism monk just east of Wild Groose Pagoda, which serve food not made of meat of animals.

You may go to the restaurant of Daxingshan Temple for a vegetarian dinner, which is located in Xiaozhai area. And, another popular one is called Su Xin Ming which is located a little far from the city center.

There are two Indian restaurants in Xian as far as I know -- Delhi Durbar and Cacaja. Both are close to each other but the former is better in terms of food quality and service. Price is reasonable and placeis clean.