The Impact (4-14-5) won for the first time since June 29 in Houston. They remained 10th and last in the MLS Eastern Conference behind the ninth-place Fire (4-6-13).
What had been mostly a defensive struggle broke open when substitute Maxim Tissot sent fullback Eric Miller in on the left side. Miller battled with Razva Cocis and the ball squirted out to Di Vaio, who calmly slotted in his fifth goal of the season from near the penalty spot.
Chicago was coming off a 6-0 loss to Seattle on Wednesday night in U.S. Open Cup play.