Although strict Cambodian Buddhists do have a vegetarian meal once every two weeks on offering days, Cambodians in general can’t understand why anyone who can afford meat or fish would not want to eat it.
The best way to get a vegetarian dish is to ask for your order to be cooked without meat (ot dak sait) or fish (ot dak trei); in principle, most stir-fries and soups can be done this way. You might be told that the dish is “not delicious” without meat, and the waiter may also come back a couple of times just to check he’s got it straight. However, to be sure that prawns, chicken, duck or even intestines aren’t substituted, or that a meat stock or fish sauce isn’t used, you’ll need to specify a whole list of things to avoid. Vegans will need to make sure that no eggs are included (ot yoh pong mowan) as these are widely used, but should have few problems avoiding dairy products, which are unlikely to be found outside Western restaurants.
In tourist centres one or two vegetarian restaurants have opened, while restaurants catering for foreigners will also have more choice and a better idea of what being a vegetarian means.