How To Summon a Vex That Fights for the Player?

Hi. I'm working on a mod that adds mob staffs to Minecraft. One of the staffs is the evoker staff, one of the abilities of which is summoning three vex like the evokers do. However, I want the vex that the player summons to fight for them, like how an evoker's vex fights for it. Is there any way to do this, and if so, how?

- Khmer