Title explains it well, Vex that are spawned by an evoker are expected to begin taking damage and die after roughly 30-119 seconds of being spawned. To replicate:
1) spawn an evoker in creative mode
2) Enter Survival mode and move around close to the Evoker until it spawns a batch of vex
3) Wait 2 minutes and observe as the vex remain in the area and do not die until killed by a player.