The word for ‘vegetarian food’ in Chinese sounds exactly the same as the word for ‘fast food’, so if you are a vegetarian it’s probably best to double-check before leaving for the restaurant.
Actually, the word for fast food is very rarely used in general conversation, and people would usually just say the name of the restaurant, such as:
So when you tell people you’re a vegetarian you can say the following:
Wǒ shì chīsù de
I’m a vegetarian
You might also add some clarification by adding that you don’t eat meat:
Wǒ bù chī ròu, wǒ shì chīsù de
I don’t eat meat, I’m a vegetarian
If someone asks you what you want to eat, you can tell them vegetarian food by saying:
Wǒ yào chīsù
The 素 in the sentence above being short for 素食餐（Sù shí cān）- a vegetarian meal.