what if the vex are an evolved version of us that stayed here too long and it ended with their demise. They sent some of themselves back in time to attempt to drive us out of the system and expand among the stars. My reasoning would be that they really aren't directly attacking humans and their tech seems to be so far advanced that they could knock everything out if they wanted. Instead of wiping out the weekend humanity on Earth. They started with mercury and are working outwards from there and are more concerned with fighting the fallen and cabal and looking at our old secrets from the golden age. They aren't here to kill us, they are here to force us to choose a different path than the humanity that resulted in the vex state.