Just want to be sure - are you talking about Acquapendente in Viterbo? is this a trip there and back, or are you looking to use it as a base?
It looks as if you would want to get to Siena first and then from Siena to Acquapendente....however depending on how many people you are, if you have luggage and if this is a day trip - you might also want to consider looking into a car with driver OR even just renting a car for the day and drive yourself.
My daughter and I are going to walk the Via Francigena from Acquapendente to Rome, starting Sunday, May 31st. Our flight from Houston arrives in Florence May 28th. We will stay one night in Florence and leave to Acquapendente Friday the 29th, hopefully around noon or early afternoon. We do not carry luggage, just our back packs. My understanding is that we have to take a train and a bus to get from Florence to Acquapendente, but I do not know where to start. I am planning on getting more information at the train station in Florence. I was also told that if reservations are required, we have to purchase the train tickets in advance to avoid paying high last minute fares.
I appreciate your input and thank you for your response.