However, if you have Vader show him the vision, you are changing the character of Vader without knowing that is the direction they are taking him in future films. Anakin was redeemed in RotJ. Having him be evil from beyond the grave seems to undo that.
My thinking was that these powerful artifacts can induce Force visions, and it is only Kylo’s supposition that Vader himself provides them. In reality, it could be a machination of Snoke or his own wishes combined with the aura surrounding an artifact that had seen evil.