Julia will be able to confirm that, as noted by the guards, the blood signature is not broken but rather an entirely unauthorized one was used in conjunction with lifting the wards in order to remove the Twin Latchkeys from its place.

There is no trail from the blood. The blood signature is similar to a key, and would not leave any trail behind. As a result, it may be best to begin searching for a match. They can speak to witnesses to do so, but be aware not all of them will be cooperative without good cause.

Guards would recommend she examine the wards if she wants to know more about the magic involved in taking them down. The wards are currently up and appear untouched, as described above, but she may learn more by looking at it closely.

The security guards would not be able to grant access to anyone's private vaults without good reason, as this would be a breach of protocol. They would suggest that if the Doctor can come up with probable cause to search one of the witnesses' vaults, then he can return and ask them again, at which point they could possibly give him access. Speaking to one of the witnesses or searching the other areas may give him the evidence required.