More than likely Travelocity is a broker of a tour company and you can book the same tour directly (for less money) with the tour company.
There are many good tours of the Vatican museums. I do not believe you will have time to make the 9:15 time without a taxi (about 25-30 Eu).
Because you are doing a day trip,I'd only book with a tour company that bypasses the line.
If it were me, I'd go straight to the forum/colloseum after arriving at termini. Then cab or bus to the Vatican to arrive around 12:30. You should be able to get in quickly. I'd then do an audio tour.