vatican city to siena

the easiest would be to take the non-stop train from Roma Termini to Firenze S.M. Novella then the bus (station is right across the street) to Siena. You're looking at about 2 hours of travel time if you coordinate schedules. The local train from Rome to Siena involves changes and wasted time. The bus from Rome is long and uses the main highways so the views wouldn't be that great (the high speed train goes by different views faster).

Kathleen, I disagree with Robert. Getting from Rome to Siena will take over three hours whether by train or bus. Going through Florence is not easier just more expensive. See http://www.trenitalia.com/cms/v/index.jsp?vgnextoid=ad1ce14114bc9110VgnVCM10000080a3e90aRCRD For times and routes.
Easiest might be the bus, see http://www.sena.it/index_e.htm

Bus is about 3h10m; train is (typically) 3h45m. An advance purchase train ticket could cost as little as €26,70 (reg walk-up charge of ~ €57,70), and a bus ticket (reg €23,00) could cost €10-15,00. Personally, we're train people, so we'd more than likely take the train (as we did last autumn...). Then, we were staying closer to the train station than city-center Siena, but we've always taken the train into Siena and just taken a €10,00 taxi to our hotel...just 'cause we can! We don't always arrive in Siena from a town on the bus line. Just peruse the info on Trenitalia; click on the English flag at the top of the page. You can search Roma-Siena, or if you want some time IN Florence, search Roma-Firenze and Firenze-Siena. (you want plain ol' 'Siena', not Zona, etc.) You'll see the hourly pattern of departures. It's always €7,70 for the train from Firenze-Siena.

Hi Kathleen, I am an American living in Siena. I strongly recommend the Sena/Baltours bus from Rome to Siena. It is direct, with three stops in Rome and four in Siena. The most central Rome stop is "Tiburtina" across the street from the Tiburtina Metro (Subway) Station. There are about eight buses scheduled per day between 8 am and 8 pm.. www.sena.it. It takes 2.75 hours. Regional buses from around the country come to this bus station so it is busy. Just walk around until you find the "Sena" or "Baltours" sign, buy your ticket, and wait. There are benches and a bar/candy shop so if you are there early you can just chill. Be vigilent about protecting yourself from pickpockets while you wait; the area is a bit seedy. Once you are on the bus, though, it is great. Luxurious, comfortable, and mine was friendly. I went in March 2012. I have also been on the train and that was ok, but more jangly as I recall. The train station is a nighmare. Our train trackbinarywas not anywhere to be found. Finally, with a lot of asking around, it was a very long (10-minute) walk to our train with all of our stuff. We almost missed it. That was October 2011. Beth Young