There are no foods that are commonly considered vegetables that begin with the letter X. In culinary terms, a vegetable is simply any edible plant or part of a plant. There are a few fruits, which are technically vegetables, that start with the letter X.
"Xigua" is the Chinese word for watermelon; in English, it the word is sometimes used to reference a small, round watermelon. The ximenia is a flowering plant native to India and its surrounding areas that produces a small orange fruit that is edible. Additionally, "xylocarp" is a term used to describe any fruit or edible plant with a hard, woody outer layer, such as the coconut.