Is the future set in stone or what?, does time traveling to the past change the present? And most importantly what the hell is up with vex sending their troops to the past/future?
The Vex Network basically exists in its own weird way outside of time, where resources can be diverted to specific times and places.
Events can be rewritten, after all the most famous example (besides the Saint thing) is the Vex intentionally allowing a transmission from Praedyth to escape the Vault of Glass in order to purge Oryx’s Taken from within as to avert their own extinction.
The Vex are more than happy to play the long game, after all outside of one instance, that being Season of the Undying, the Vex themselves have never gone on the offensive against the forces of the City.
Unfortunately though with a lack of a central villain after the death of Quria the Vex are going to be a challenge for Bungie to do anything with. Although the portrayal of the Network through the fake Maya in the Clovis Journal gives me some hope.