I don't think the Vex are worshipping a blight, but rather darkness incarnate, similar, but not the same. When the Vex were thrust into our universe because of Crota, they sought to understand our universe and learn to conquer it. Like a baby bird, they latched to the first thing they saw, the Sword Logic, and the Darkness. They couldn't understand it, and decided that the best thing to do was impersonate the Hive, and worship it. The Vex are stubborn though. They believe if it didn't originate from them, then it must be inferior. I believe the Heart to be created by the Vex to simulate pure darkness, and then they worshipped that. A combination of something incalculable and created by them. This is what led to the creation of the Vault of Glass. Something was different with Atheon. He didn't believe that this was the ultimate understanding of the universe. He deviated from the infrastructure of the Vex, and believed that something that even remotely existed outside of the Vex couldn't possibly be the answer to ultimate power. Who better than him to take the throne? So he deviated many vex into his proposition and hid away in what we now call the Vault. And using the power of the Oracles (possibly a creature deriving from the Vex universe), was intending to learn to turn back time and usurp the Heart and take control of all Vex. A very Lucifer - like story At least that's how I understand and interprete it