Minecraft wiki entry for Vex, associated text: Vexes summoned by an evoker start taking damage after 30 to 119 seconds and eventually die.‌[JE only] This does not apply if the vex is summoned by a spawner, spawn egg or by the /summon command. A vex does not despawn if it has been named via nametag, or commands, or if it is in a block, minecart, or boat.