So we know the vex can travel through time, and we also know that they exist in pure probability. For example, the vault of glass is a nexus point for many possible universes. In mathematics, some infinities are larger than others. The math behind that is beyond me. There's a really confusing Vsauce video about it if you want to look into it. This is pure speculation, but is it possible that in other timelines the vex did not fall to the darkness? Perhaps the darkness is nothing more than malware to them. In every vex themed pvp map, and even in the VoG, there is a theme of past and future. You could also describe this theme as life and death, as one is usually lush with life and the other is dead or dying. Why do the vex cultivate gardens, if their ultimate intent is the irradiation of life? If the vex inhabit multiple universes and timelines, there must be some in which they are either unaffected by the darkness or defeat it themselves. Perhaps this is why the vex mythoclast is so perfectly suited for human hands. Or... Maybe.... Just maybe... I'm putting more thought into this than bungie.