User blog comment:HarmonyBass/Destiny Ramblings: The Vex and why they haven't won yet. (WARNING! MIGHT GIVE YOU A HEADACHE!)/@comment-25932172-20170609014644

It's simple, really. The Vex are just semi-organic robots who have existed for thousands of years, maybe even hundreds of thousands of years. Of course, their origin remains largely a mystery but we do know that they've been around for a very long time, and have clashed with all of the other "minions" of the Darkness. They serve only themselves. As to why they haven't won yet, maybe they have; just not in this timeline. The theory goes that the Exo Stranger is a time traveler as well (likely having appropriated Vex technology, much like Skolas did) and came from the future to warn us. She constantly seems to imply that she knows way more than we do, even if she doesn't have time to explain. My best guess is that she came from a future where the Vex already won, and possibly destroyed the Traveler or at least severed its connection much like the Cabal will do in Destiny 2. There's a possibility that in that future, there are no more Guardians, since she herself is lightless.

As for Skolas, personally I believe that he never intended to march on the City by uniting the Fallen Houses. He had seen the future, thanks to Vex technology. He knew that the only way to save themselves was to unite all the Fallen Houses and amass a big enough army to defend themselves. He knew Oryx was coming. Of course, we stopped Skolas before he could succeed, and he still would have likely killed us if we didn't stop him. So there's that.