I think unless you really want to be in Chicago (and honestly, I can understand, wonderful city), I would throw all of that Vault crap away. If you think you are wanted and will thrive, and you like the place, I think there is no reason for you to change. It actually sickens me sometimes how much weight people put in rankings sometimes, especially with a career. Who cares if you are at a top Vault X firm and what your exit options are if you like where you are? (BTW, I'm not saying that is what you are doing, I'm just saying people put more weight in Vault than I do).
If you want to be in Chicago however, then you have a real choice on your hands.
I'm considering a split and have the same concern about damaging reputation, however, if the small market firm is interested in offering you a split, that is a good thing for them. As for the other firm, I understand your pain.