Vexes are supposed to be ruled by evoker. But it attacks unordered targets, which is not consistent for players and mobs. 

Simple video for explaining it

https://www.youtube.com/watch?v=9Rh–oQGeMM

How to reproduce:
make closed space and summon evoker and wandering trader inside of it.
summon wandering trader outside of it.
evoker would summon vexes to attack wandering trader.
wandering trader outside of evoker's sight does not get attacked, but player would get attacked.