The vex could go back in time and kill grandpa Gohan when he finds goku as baby and then just kill goku so..
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Why couldn't the vex kill us like that? We win against them so I'm sure someone else would too
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Actually. The golden age scientists theorized that the reason the Vex can't go back in time to kill them is because at some point in our timeline we were in some way involved in their creations and any alterations to that would create a paradox.