From the Gimoire:
Survivors of the Vault of Glass report sightings of ancient Vex - ancient in the sense that they have endured for eons. Convergent analysis from multiple Ghosts suggests that these Vex exist in our future.
If the Vex exist in our future - or in a possible future - should we take this as evidence that their defeat is impractical or unattainable? The Guardian Vanguard is quick to point out that time travel remains a mystery, and that the continued existence of the Vex is not remotely a sure indication of humanity's extinction."