With regards to Postmortem's post regarding the Vex tracking something...

Above the map in the Nexus, many small lights can be observed. I thought they were just data clusters that simply represented the Vex's computer-like information processing...

But I saw something else inside the Vault that caught my eye...

The Aegis.

The one tool vital for beating the vault, it seems to have followed the Vex into the past and future, while originating from the present (according to the Grimoire).

Given that you find all three versions of it (past, present, and future), it stands to reason that the Vex have always been unable to obtain it, and know that it is the wrench in their works, since Kabr was somehow able to manipulate their time travel, and mold it into an object that he fashioned within the Vault.

If they know this in the past, then why didn't the Vex level the city on Venus? Why didn't Sekrion turn Venus into a machine, like the Vex quickly did to Mercury? Could it be because he's waiting for Kabr in the present? But that's where we first find the shield, during the present-day Templar fight... then again, the shield comes from the cleansing platform, and who knows what that's linked to. Sekrion may have been scanning the planet for ages, searching for the shield's point of origin...

On a side note...

In the Grimoire, Kabr says that they took his Ghost, and you do find one Ghost inside the Black Garden. Could they be the same?

According to our Ghost, The Black Garden is apparently on Mars (unlike the Vault in Venus), but in a different point in time. The Vex, however, don't resemble the Descendants that we find in the vault, so at what point in time does it exist? Kabr has already been there, if the dead Ghost is anything to go by, so would the shield be from their future?