Tickitaly can reserve your tickets for the Vatican. But not for the Scavi Tour. Read on.
As with requests to take part in a Papal Audience, gaining a place on a Scavi tour also takes some faith. And patience. But that seems only right. There is slight confusion as to how often the tours run, how many people can take part and how long they last; the following two quotes both come from the Vatican’s own site (the first from this page and the second from this page):
Visits are organized according to the schedule set by the Excavations Office.
In order to preserve this exceptional historical, archaeological site and due to the limited premises around the venerated tomb of the Apostle Peter, only around 250 visitors per day are permitted to enter.
Due to the need to give careful attention to the conservation of this irreplaceable historical-archaeological site, wherein is preserved the original Grave of St. Peter, only about 200 visitors per day are permitted in the necropolis.
Some people report a tour of 45 minutes, other double that. There are also reports of group sizes of more than 15 people, so it is clear that things are flexible. What is equally clear is that you need to organise these tours yourself:
On account of the significant and repeated difficulties that the Excavations Office has had with third-party requests for tickets (i.e. through travel agents, etc.), the Office will only make reservations directly for those individuals who will actually be visiting the necropolis.
So fire off your request as long in advance as possible, and hope for the best. Responses are by all accounts quite fast, and you can find details of the full procedure, costs and regulations on this page. Good luck!
The official Vatican site now has a very rich and detailed virtual-walk through of the Necroplis and St Peters tomb, as introduced in this video: