Rather than Light = Life, Darkness = Death, and Vex =Limbo, I believe the dynamic to be closer to Light = Creation, Darkness = Destruction, Vex = Time and/or stagnation. The Vex are capable of Time travel on a massive scale, and any race capable of time travel would be able to defeat any race incapable of time travel in a war. The Vex, by all accounts, should have already succeeded in conquering all of existence because they can time travel while no other known race can. This raises the question, "Why haven't the Vex already conquered the universe if they can time travel?". The answer is clear to me. The Vex haven't conquered the universe yet because it's not their goal. The Vex don't need to, or even want to, conquer anything, at least not in the same way races like ourselves or the Hive do. The Vex simply desire to be a law of the universe, ingrained into the very fabric of reality. To that end, all the Vex need to do is wait. Wait and calculate. They don't want to participate in the conflict between the light and the dark, they only care about the outcome. The Vex want to see which side the universe favors, because it will give them that much more insight on how to become a universal law. The Vex only need to defend and wait, that's why they have converted into robot bodies that cannot die of age. The Vex hardly even attack us, they almost entirely take defensive action against guardians. We arrive on already mechanized worlds such as Venus, Nessus, or Mercury, and the Vex defend their holdings. The only time we ever see the Vex attempting to claim new territory is on Mars in D1, but once we kill the mind leading the Vex on Mars they seem to give up and only defend the small parts of the planet they managed to control. Really, I don't think it's possible to defeat the Vex when they hold both the advantages of time travel and only needing to defend. The best course of action is simply to make it so that the Vex need us in some capacity to complete their goals. If ever given the chance, it would be ideal for us to become an ally of the Vex. We might just be able to join the Vex at the end of time if we do.