Sorry if everyone knew but anyway think about it. The Exo Stranger in the Vanilla story wants you to destroy the black garden. WELL now it makes sense she's Vex! But why would she betray her own team? Well, basically it's like a time loop, if the Guardian doesn't destroy the Black Garden, they won't be powerful enough to defeat Oryx who will obliterate them so really, the Stranger works for the Vex because they were defeated in the future so learnt from their mistakes so went back and encouraged the Guardians to win. Makes sense innit?