After deeply diving into the grimoire I have come to the conclusion that the Exo's are simply corrupted units of the vex. The vex themselves are sentient, self aware robots that move as a single unified unit. The Exo's are also sentient, self aware units who have no memory or recollection of what they are/who they are. My conclusion is that they are units who wouldn't conform to the vex network. After overtaking Clovis Bray facilities the Vex began to fully understand the Exo's body type and conscious being (alterations the humans made). Where did Clovis really begin to understand this type of technology? The Vex structures on Venus. This leads me to believe that they were planted as to gain an advantage over guardians and begin to simulate and understand light as a whole. Now once they have an understanding of how the alterations occurred the Vex attempted to absorb the Exo's into their network, some may have been taken into their network and thusly made into minds. It's how the Vex knew to use Praydeth to save themselves, it's how the vex know we can stop the darkness, and it's how the vex knew that invading us would lead to their termination. Now as for the Exo's that are still on our side were attempted to be absorbed by the Vex, when they broke free they lost their memories as the Vex went and absorbed their memories/knowledge and then attempted to remove that from their Exo hardware. After resisting the absorption they lost their hardware memory, but they had their consciousness to remember which is why when they're restarted their programming can better grasp the memories. It's why when they go behind the veil they can find more memories but it endangers them due to their body not being able to handle that mental strain. It also explains the Exo's hatred for Vex as shown by the Exo grabbing a leg of the Vex and starts beating the Vex with it. They're locked in a war and the Exo is subconsciously letting go of their hatred whilst fighting them. Thanks for reading! Tell me what you think.