I believe that I saw an image of early concepts of the game in with there were 5 slots for baddies instead of the four that we know. In that case, you may be correct in the Vex modeled after other beings theory. Secondly, I agree that the traveler may have offered its power to each of the species but, as of yet, they turned on the traveler and tried to abuse its power/ harvest its tech for their own use. The Vandals are basically scavengers and pirates, so they would want to scavenge the traveler. The Cabal are war-like and would want to weaponize any new tech they can get their hands on. The Hive might seek the Traveler's power of revival to return themselves to their original states. The VEX might.......... well... maybe they are trying to stop us terminator style from abusing the traveler ourselves like the other species. The Vex may even had been made by species who inhabit the next world that the traveler goes to in attempt to stop humanity from attacking them in an alternate reality or etc.