Board Thread:Destiny Forum/@comment-69.228.33.19-20140721002858/@comment-96.250.89.146-20150127165924

Although the Fallen and Cabal were not made from the Vex, or tied to them in any way. They were simply manipulated by the Vex, using their Oracles to rewrite their history. In this fashon, they placed in soldiers of Vex orgin to force the Cabal and Fallen into their territory war to reclaim the dormant planet AIs in the solar system. They still fight with them because the Vex on Mars and Venus are different Collectives, therfore, are individual from eachother, not manipulating their minions back into line. In fact, those Collectives couldn't if they wanted to, for they would not be able to insert operatives in the command structure of the enemy. Although I wouldn't be suprised if in the next Destiny game, we will see Vex, posing as humans or other living beings.

~Phantom of the Wiki