You are able to spawn Evokers and Vindicators in peaceful difficulty but not Vexs. Therefore, It either WAI for the ability to spawn Evokers, Vindicators and Vexs in peaceful difficulty or they should NOT spawn at all, because they are hostile mobs just like Zombies and Creepers.
MCPE-21399
You can spawn Evokers and Vindicators in peaceful difficulty but not vexs
Resolved
Works As Intended
0
Since Evokers/Vindicators doesn't respawn, that's why you're able to spawn them on peaceful difficulty. However, since Vex are able to respawn, they cannot be spawned in peaceful. This is working as intended.