Best way to travel from Rome to Venice;
From Venice to Vernazza;
From Vernazza to Sienna;
From Sienna to Sorrento; and
From Sorrento back to Rome?
Train all the way. Rome to Venice takes 4.5hrs. Venice to Vernazza takes 7.5hrs. Vernazza to Siena takes 4.5hrs. Siena to Sorrento will take 7.5hrs (including the private Circumvesuviana train from Naples to Sorrento) and finally, Sorrento to Rome takes 3.0hrs. There are no airports close to Vernazza, Siena or Sorrento.
One alternative for the Vernazza to Siena segment is to take the train into Florence and then take the SITA Rapide (Express) bus to Siena. The bus takes you to the city wall. The Siena train station is about 1km uphill from the train station but there are frequent local buses to the city wall. Your choice.
We flew from Rome (CIA) to Venice (Treviso)it was less expensive than the train (Ryan Air) but I think ended up taking just as long. If you can get into Marco Polo airport for a reasonable price it would be worth it :)
I would vote for the train from Venice to Rome (not familiar with the other routes), mostly because I think train travel is much more pleasant than flying, at least in Italy. Trip takes about 4.5 hours, wonderful scenery en route. If cost is an issue, save money by traveling second class (the difference in comfort between first and second class is negligable) and get the Amica discount by buying your tickets a few days in advance. While the actual trip is longer by train than plane, you save long trips to and from the airport (train stations are right in the middle of the city) and don;t have to check in as early.
Thank you all for your responses! It looks like training it is the way to go!!