Orvieto is wonderful, but visiting it this way would put you there for a relatively short time in the middle of the day, when it is most crowded with day trippers. In my opinion, it is nicest early in the morning and in the late afternoon and evening, as is often the case with popular places. Instead of taking a fast direct train from Florence to Naples, you would likely need to take slower trains from Florence to Orvieto, and then Orvieto to Rome and on to Naples/Pompeii, often with connections. So, considerably more travel time and hassle. You would probably also need to check your baggage in Orvieto, which I believe cannot be done at the RR station, but at a nearby hotel. Personally, given all this, I think I would leave Orvieto for a longer stay during my next trip to Italy. Also, be sure you understand where your hotel is located in Pompeii, and whether you should take the regular train from Naples to the Pompeii city stop, or the Circumvesuviana train to the Pompeii Ruins (scavi) stop. There is some distance between the those two lines and stops, I believe.