Plan on taking train from Orvieto to the Vatican. There are 3 train stations in Rome. Does anyone know what station is the closest to the Vatican ?
Termini is most convenient, although Tibertina would also be fine. You will probably take the metro from either one to Vatican City, use line A, stop at Cipro.
There are lots more than 3 stations in Rome. Where are you staying or just going for the day? You could pick Roma San Pietro which is just a 5 minute walk to the Vatican. For this you would change trains at Roma Tiburtina.