As far as I've learned: lagar=cook, mat=food, so lagar mat=cook food. Though "food" was often omitted in other exercises. "we cook vegetarian" doesn't sound like a full sentence to me, non-native English speaker. However, it is a sentence that could be used in everyday life in Lithuanian.
To begin with, it's not correct English to say "We cook vegetarian meal", you'd need to have an article in there: "We cook a vegetarian meal". Secondly, a meal is en måltid in Swedish. Those words refer to the food you eat in one sitting. When you say Vi lagar vegetarisk mat or We cook vegetarian food, that can refer to something much more general, like you cook vegetarian food every day or always, not just the once.