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.

Do mobs still spawn in peaceful?

Unfortunately, spawners can't spawn mobs on Peaceful difficulty, so in Peaceful, you can't listen to the sounds of zombies, skeletons, or spiders to find dungeons.

What happens if you name a Vindicator Johnny?

It is that when a Vindicator is renamed "Johnny" with a name tag, it will become hostile towards every mob, but they will become passive towards you and all the other players. They will not attack players but will attack other mob when renamed as "Johnny".