You do have a great itinerary for your first trip, but I do have a couple of suggestions. I usually wouldn't suggest a daytrip to Pompeii, but if seeing this sight is the only reason you are traveling this far south, then I would recommend it in this case. If you go on the daytrip on your second or third day in Rome, you could be saving a night to use somewhere else (i.e. you wouldn't be staying in Naples, and you can use your last day in Rome to explore Rome during the day--which still gives you your 3 days in Rome overall--then train to nearby Orvieto in the late afternoon/early evening).
With this extra night, are you looking to explore Tuscan towns? If so, I would suggest adding the extra night to Lucignano. This way, you aren't staying there for simply one quick night. Also, you would have more options for daytrips to nearby Tuscan towns since you will have your car at this time. Your options I would recommend would be: From Orvieto--Civita di Bagnoregio and Todi; From Spello--Assisi, possibly Perugia, or, if you like wine, the small town of Torgiano; From Lucignano--Montepulciano, Pienza, and nearby Cortona across the valley; and From Siena--San Gimignano and Volterra.
You would be able to see an extra town or two with this extra night you would have in Lucignano. I would also recommend (as suggested before) that you go to Vernazza after Siena, then come back to Florence before going to Venice. This way, you won't be wasting nearly a full day traveling from Vernazza to Venice.
However, (starting over) if you are interested in the Amalfi coast, which I would also highly recommend, then I would take your two nights in Spello and add them to the Amalfi. This would give you 3 nights down there which would be perfect to base in Sorrento and see Pompeii, the Amalfi coast, Capri,etc. With this you wouldn't see as many Tuscan towns, but you would certainly still get a taste of Tuscany.
I hope this isn't too complex, but if you have any q's let me know.