Polish food, built as it is around pork and game, is not particularly vegetarian-friendly, though there are plenty of meatless items on menus that you can build a meal around. Pierogi "Ruskie," for example, are stuffed with potato and cheese, but be sure to ask them to hold the bacon bits they sometimes throw on top. Other popular meatless pierogi fillings include cabbage and wild mushroom.
Green Way (www.greenway.pl) is an inexpensive and excellent chain of vegetarian restaurants with branches around the country, including in Warsaw, Kraków, Katowice, Gdansk, Poznan, and Lódz.
For more vegetarian-friendly travel resources, go to www.frommers.com/planning.