Since we are useful assets to them, since we got tricks into clearing the vault of the blight.

Guardians are paracausal and vex cannot understand and defeat paracausal entities. Guardians and the hive are prominent paracausal entities so, either the Vex will bite the bullet and assist the guardians in taking out the hive and taken so they can survive long enough to further their own gains, or, the hive deal with the vex for us.

Consider this. Who would have saved them from the taken? They have big plans for us and I also don't think that can get too close to the traveler for some reason. Light and whatnot.

Because it's not part of their plans...yet, it's very well possible they have beat us already and we don't know it yet, enter the exo stranger, but that's just my two cents.