I just registered to share my ideas about what it is that is chasing the Vasari (i really feel sorry for dem)
Option 1) the Greys. Well you know, the greys!
Option 2) psionic slaves.
Option 3) something like the Reapers (ME)
Option 4) some sort of life but not as we know it (i.e. living cloud!)
Option 5) orks!
Anyways, I would not stick too much to the fact that the adversary drove Vasari insane. I do not think we know enough about Vasari psychology to draw any conclusions. As far as we know, it might be some sort of Hello Kitty-product advertising campaing.