If you look at some rocks they are cracked open and have tec in them. Like boards and wires. Also the vex change change history, but they can only effect things so much, they can't go back to eradicate something that was the sol purpose of eradication. It'd be a paradox so we still exist cause they fear if not they never would exist. Plus we killed the heart so the power over changing time is greatly weakened. And certain strike bosses. But basically they can go back in time to effect only that thing and not our history of it. They in them selves are a paradox. They use and reuse units throughout time. You might even have fought the same vex unit before and never known it.