Dude, I'm telling you, Skynet went on a week-long bender, got it on with a Cylon & decided to make dinosaur-headed babies, which she sent back in time to kill the assclown that kept buying her beers that night. Obviously she's still drunk, otherwise she'd realize that attempting to alter the events that lead up to her intoxication & subsequent impregnation is impossible due to the phenomenon known as a Predestination Paradox, which ultimately and invariably leads back to the situation that prompted the time travel in the first place. I believe that the campaign will cover the player's Guardian battling its way through the Citadel in order to confront her with the reality of the situation & convince her to suck it up & start acting like a responsible parent... Most likely after liberal applications of coffee and aspirin. In all seriousness, I suspect that we will find that the Vex were ultimately either created by, or were influenced through, Humanity's experiments at the science colony on Venus.