If I buy a ticket from Monterosso to Venice, the transfer time at Milano Centrale is only 15 minutes. I believe that the train from Monterosso to Milan is classed as a regional one. The one from Milan to Venice is a fast train. I have read that the trains to and from Monterosso are notoriously late but I'm not sure if this applies only to the line that connects the towns of the Cinque Terre or if this applies also to trains heading for Milan. If a 15 min transfer time is not realistic, I think that I could book two separate tickets, allowing a transfer time in MIlan of 1h 15 min--a longer but safer layover.