I think you missed. Your focusing on the death of the star system....and you're largely correct. But the "dark future" that the Vex were trying to simulate and find a path to is about THE HEAT DEATH OF THE UNIVERSE. IOW, the Universe was born in the Heat of the Big Bang...and has been expanding...and cooling...ever since. Trillions of years from now....probably on the order of 100 Trillion years...the Universe will have expended all of the available means of generating energy...and it will come to an end. In the cold...and in the dark. If you read the Lore---Especially The Books of Sorrow---its suggested that the Conflict between Light and Dark is about how to best come up with some form of life that can survive---essentially----the Death of the Universe. The Hive and the Vex are both trying to be THAT form of Life....but they are going about it in different ways. The Hive are applying Sword Logic...and are basically trying to MURDER the competition. The Vex are trying to SIMULATE and MANIPULATE their way to it. By simulating all possible Futures...and then manipulating events to produce that Timeline. What Osiris saw was the "dark future" where the Vex had accomplished their goal. Where the Light was gone...and all the other serrvants of the Darkness had either killed each other off....or died out. So only the Vex were left standing. AT the heat-death of the Universe, pretty much everything is going to be a cold "black-body" object.