The Kopper Kitchen is a classic American diner, but there are a few vegan options, which is refreshing. Pretty much everything listed as vegetarian on the menu has cheese, but the staff is very accommodating and will make items vegan on request. There's a vegetarian special on Tuesdays, which I believe changes from week to week. When I had it, it was spaghetti squash with marinara, which was very good. The tofu paté that's used in most of the vegetarian entrées is a little bland, but it's a nice effort and can be improved with just a little salt.
Overall, it's a fairly-accommodating American-style diner with several basic options for vegetarians and vegans.
Cons: Not many options, Food is a bit plain, but still good