Last time I was on an itinerary like that, IB decided to be horribly late, effectively killing my layover and ability to tour the city, but for the 19:45 departure you should be OK

We went to VCE last month, flying ORD-JFK-MXP and then as we were going to be touring a bit, drove the last leg to VCE. Next time I'd opt for the train - travel time is about 2:30, and the station is right on the Grand Canal (easy access to some hotels, the water bus network, and water taxis). Time it right, and you could be settled into your hotel in the early afternoon, and enjoying dinner on the canals vs. sitting in a lounge in MAD.