The Timestream connects the past and the future of the Vault of Glass. This is presumably intrinsic to the Vault of Glass and is what allows the Vex to move through time. In the Timestream, the Vex are able to look at all possible outcomes of the situation they may be faced with. If the Vex encounter an outcome that is undesirable, they can adapt accordingly and avoid failure. Atheon, Time's Conflux sends Guardians through the Timestream to separate them from the Aegis and weaken their ability as a team.