Would like to hire van and driver for 6 people from Viterbo to Florence in October

I would start by asking your hotel for recommendations. For 6 people plus luggage, you will need a full size (8 person) van.

Google: "NOLEGGIO CON CONDUCENTE VITERBO"

Then see what comes up

I would suggest you to contact a local tour company we used in the past
www.shoreexcursionsinitaly.net that in addition to shore excursions also provides tours from town to town.