So... through the calcified fragments we have found that the Vex currently exist in this time (or became interested in this time) because Crota ripped open a portal to a different space and time that they crawled through. Nothing else is known for [i]sure[/i] so far so I want to hear what you all think below. My theory? They were imprisoned away in a time and place that had been locked out of reality by humanity in either the future or past. We know that places like that exist due to the mission you research up on Gatelords. Ghost: "Protect the realms the Vex keep locked out of time. Time? That can't be right..." And now that the bottle is uncorked they've spread through time and space like wildfire but never interacting too heavily with the past in order to ensure their release via Crota and thereby becoming a paradox.