The vex module of "seeing into and simulating the future" is flawed and based of relativistic, computable rules. It doesn't take into account quantum improbability. Knowing the path and location of everything doesn't yield ultimate knowledge because, like our characters, there is a layer of cruel forces and RNG that can't be reconciled. The "light" defies everything we "know" about physics. When you are in the Vault of Glass, everything runs according to how they think reality "should" work. Guardians, aided by the light, can manifest their own destiny and will: "guardians make their own fate" (because our power comes from a force that can manipulate quantum improbability) So when a guardian is acting under "times vengeance" he is literally acting outside of the Vexes bastardised "understanding" of time. Basically, The Vex can't comprehend, mathematically how you're kicking their ass, but you are.