Q:

How do you describe a vegetarian item? As in a vegetarian burrito?

So, in english when describing whether something is friendly for vegetarians or not we say "vegetarian object_____". Is there an equivalent in Chinese? 
For example could you say "素的墨西哥卷“？

A:

In Mandarin, we use the prefix "素".
In Cantonese, we use the prefix "齋"
And the order is significant too:

"素墨西哥卷" implies that the original burrito contains meat and this one is a customized version for vegetarian.
"墨西哥素卷" implies that this vegetarian burrito is original Mexico food.

On the other hand, as wuyefeibao mentioned, you can add "的" in the phrase; and it is quite interesting in this case:

"墨西哥的素卷" == "墨西哥素卷"
"素的墨西哥卷" .... It can be both.  ^_^

