Here is my thought, what if the Advent are drawing on the power of a idea? The idea is Justice, pure and unrelenting. Then over the years they started to turn to Vengeance for power to fuel the psychic abilities.
What if the Vasari are running from a similar "idea". Like they stumbled onto pride, or arrogance maybe even the embodiment of fear and it started to slowly cull the minds of the Vasari and slaves alike.
It would explain the insanity, it would explain the running, and it would explain the corruption of the wholeness of the unity. I imagine kind of like the gods of chaos from the Warhammer universe, how the emperah created order and as such the universe saw fit to balance that with creating the gods of chaos, kind of like how the law of entropy works?
Because it seems all the races hold some "similarity's" in emotion, maybe it's a power of the universe and all living being are subject to what it does? They do take lots of ques from Star Wars it seems, so maybe it's along the lines of the Force?
There is lots of power in a idea. It can rule a empire or destroy it, and it's impossible to destroy, only suppress or to hide. A idea follows the same rules of energy I say, ideas are never destroyed or created simply "discovered" maybe the Unity is the idea made real as how energy is made real into matter. Maybe the Vasari are running from such a "idea".
that's my 2 cents hope you like it :3
TL;DR Vasari are running from a idea made real, similar to how the Advent are Justice/ Vengeance made real.