Yajuu! is a hip cafe/juice bar that is located right on the Plaza des Armas. There are at least 20 types of fresh fruit juice on the menu, plus you can mix your own flavors by combining fruits. All of the vegetarian sandwiches were easy to make vegan by leaving off the cheese. Yajuu! also makes some of the largest, most delicious-looking fruit salads that I have ever seen.
I didn't have a chance to try the vegetarian sandwiches at Yajuu!, but I loved the fresh juices. My favorite was the watermelon juice (jugo de sandia), which was made from very ripe, flavorful watermelon. While the menu states some of the items in English as well as Spanish, it's important to order the watermelon juice in Spanish - otherwise, the waitstaff assume that you want "jugo de melon," i.e. cantaloupe juice (this happened to me twice).
I highly recommend this place for a quick, refreshing drink or snack. The service is very prompt and the prices, while expensive by Peruvian standards, are quite reasonable for US tourists (approximately $2.50 US for a huge glass of fresh juice).