14 Day Trip: Venice Siena Rome

Starting in Venice, three full days would be wonderful, with the first recovering from your probable jet-lag. You could have a nice day-trip to Padua (Padova) or Vicenza, both nearby and serviced by frequent - and cheap - regional train.
Siena needs two days to enjoy. You could easily add on one day for Florence, which can be done with a frequent bus connection.
Rome has so much on offer that four full days are more or less the minimum. Day trips to Orvieto (a wonderful hill town), Tivoli (the gardens of Villa d'Este) or Ostia Antica (Roman antiques) are easily done.
The Cinque Terre or the Amalfi Coast are no realistic day trips, being too far away.
An extra destination could be Bologna, halfway between Venice and Siena, which is also a good base for nearby interesting towns such as Ravenna and Ferrara, but it would give you less time for your main interests. Keep it for a next visit.

First, count nights on the ground rather than days, so you don't count departure day (probably arriving the next day after a night on the plane, don't count it). Your last day will probably be used up getting to the airport.

In my opinion, three bases is plenty, and you've chosen good ones. If you do 4-4-5 nights, you can add in a daytrip for each base. I would do that rather than pack/unpack-checkout/checkin one more time.

What are you interested in seeing/doing? Food/wine tours, vineyards, museums, churches?

3 days Venice (add a day trip somewhere)
Take train to Florence 4 days (include a day trip to Cinque Terre)
Take train Florence to Siena
2 days in Siena
Take train to Rome 5 days in Rome (maybe include a daytrip)

Save the rest for another trip

Pompeii is a long day trip from Rome. Instead, visit nearby Ostia Antica, Rome's ancient seaport with ruins contemporary with and somewhat similar to Pompeii. The Amalfi Coast is too far and the Cinque Terre are often overrun with tourists and (in my opinion) overrated. From Siena you can day trip to Lucca, San Gimignano, Pisa and Florence.

How many nights do you have in Italy? You really need to count nights, 'cause 2 nights = 1 full day. You lose about 1/2 day each time you change locations. Also it's not clear if 14 days includes your arrive day (usually leaving you only a few short hours to enjoy) and your departure day (which is a non-day unless your flight is late afternoon or evening).