Public transportation from Siena to Soriano Nel Cimino

I'll be arriving at a Diamond Resorts timeshare in Soriano Nel Cimino (near Viterbo) from the North, following a week of travel. Specifically, I'm heading there from Siena., and I have already booked all my lodging. I've spent a few hours looking for a train, a bus, or a train/bus combo that will get me close enough to take a taxi. Has anyone done this leg? I can't find a solution, and prefer not to rent a car.