The vex don't have a permanent stay on the moon, the fallen do, and they are the scavengers, not the vex, so it's only right that they cover almost all the sectors. The vex weren't even on the moon when the dlc dropped, they came when the raid opened, almost one week later. Now if you are complaining because you need vex kills on the moon, then search for the vex invasions! there is certainly 1 every 10 minutes or so in one of the 3 possible places.