Not sure about vegetarian, but, as my husband used to work at the Norwood Park Fire Department, we used to eat at the Warsaw (Old Warsaw Buffet & Lounge, ) a study in Liberace kitsch and Polish food. I assume the 4-5 cabbage salad options are vegetarian one way or the other, possibly the pierogies as well. I wouldn't say it was fabulous, but it's very convenient since you don't have to wait for table service.
We enjoyed ourselves, ate at our own pace, and when it was time to hop on the plane, hopped up Harlem to I-90 and then the two exits to O'Hare, about 10 minutes or so if the lights are against you.
Old Warsaw Buffet & Lounge
4750 N Harlem
Harwood Hts
708-867-4500