Margie, expecting that you will be on a Eurostar or other train that requires seat reservations, if you buy a ticket from Venice to Florence and get off at Bologna, the remainder of your ticket is toast when the train departs and you are not on it. Your Eurostar ticket does not permit hop off/hop on priveleges. What you will need to do is buy two sets of tickets; one from Venice to Bologna and a second set from Bologna to Florence.
For the first set from Venice to Bologna, you will most likely be on the Eurostar. For the second set from Bologna to Florence, the Eurostar is also great but you also have the choice of a lesser-class Reginale train. Here's the difference. For the Eurostar, you must have a definite date, time and seat number on the ticket. If you buy in advance (i.e. Venice), you will have to be back at the train station to meet that specific train. Or, if you are adventureous, just return to the station when you are ready, buy your Eurostar ticket for 18.10Euro and jump on the next train. Don't fret, there are 35-40 trains a day going from Bologna to Florence.
Finally, you can ride the Regionale train for 5.40Euro. No seat reservations permitted. It's the local. It takes about 30 minutes longer but you don't have the strain of having to meet a specific train. Just show up when you are ready to leave Bologna and hop on the next R train.