'Vatican Cameos' is a phrase used in BBC's Sherlock when Sherlock is about to open Irene Adler's safe. The phrase first originated in World War 2. It was used when a non-military person, who was armed (gun or knife) entered a British military base. The phrase was a signal for everyone duck out of the line of fire. Sherlock knew that John, being a military man, would recognise this phrase and duck out of the way of the gun in the safe. It is not a code phrase which Sherlock and John coordinated as a safe word. Very clever, don't you think?

When you eat dirt for the first time and leave your family to build a shack in the woods

In the Sherlock Holmes books it refers to one of Sir Arthur Conan Doyle's stories, in which he refers to another case Sherlock worked on to do with some Vatican Cameos.
In the case of BBC Sherlock it is code for duck, as in get out of the way. It probably refers to another case Sherlock and Watson worked on not shown in an episode, in which only Watson knows to duck out of the way of, in this case, an oncoming bullet. Most usually used by Cumberbitches.

Vatican Cameos is what Sherlock Holmes, from BBC Sherlock, shouts to John Watson when he’s finished opening Irene Adler’s vault and realizes it’s a booby trap (s02ep01). When in danger Holmes says a cue so Watson knows how to react properly. It’s their “safe word”.