What if the VEX can only travel forward in time? That way we eliminate the possibility of them knowing our every move and being omnipotent. Just a thought. (To clarify what if they're like the Time Machine in Futurama, it can only move forward in time- to prevent PARADOXES.)
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Robots from the future. Doesn't create paradoxes when we encounter them. Obviously they never interacted with our humanity to begin with and were created outside of Sol.