Conjecture: Vex hardware doesn't time travel, just the data/software. That's why they're time-travelling [i]robots[/i], not organics; the energy needed to send just information back and forth through time would be more reasonable compared to large amounts of matter along with said information. It's carried by some kind of wave/particle that can warp, go FTL, whatever. So, some you kill might dramatically transmit themselves through time back to their origin as their hardware ceases function. Probably through the eye somehow, like how BadVampyre13 said, projecting a beam up out into the depths of space, toward some interdimensional relay network. Just a theory; that's how I would write the Vex anyhow.