I will begin saying very good job because it's undoubtely a great post, only to say that your assumptions about the Vex are a bit stretched. To begin with the Vex are millions of years older and millions of years in the future than our guardians: unless the Traveler had come to our solar system in the remote past this would be impossible, and we have news about the Traveler only about 100 years previous the Collaps. We found the Traveler on Mars at our present time and only later we built our planetary empire and "Gardened" wordls that were impossible to live in before us: the we are the gardeners. The Vex were the Gardeners of the black garden only because they found the Darkness laying dormant in it and began to worship it. Think about it the black garden was suspended in the space time but it came obviously from Venus, not from Mars that is completely different in morphology, only the portal was on Mars. This could be for protection of the garden. Then the Vex aren't created by the Traveler, I would say that they are created by the same creator, the Traveler and the Vex are brothers.