I would say just buy the ticket in Venice - the train ride to Bologna is two hours on the cheap train, the regionale, and there is really no need to get a ticket in advance. Just go to the train station and get the ticket there - you will always be able to buy a ticket for the regionale, since there are no seat assignments, and most of the time, except for holidays, Carnavale, etc., it is easy to get a seat on the EuroStar as well, if you prefer that.
I must have been lucky - I just signed up and bought a ticket with no problem whatsoever. Regular US credit card (Visa) and no virtual credit card number. Now I have the ticket for my first train ride during my April trip. The rest I will buy when I get there.