I don’t think I could help with the voice, but I might be able to help with the VFX. I think if your going to do this right you are going to need original audio and not just splice together audio from the other movies/tv shows. You might be able to reach out to someone who can do a good impression on YouTube. Someone not looking to monetize their channel. That’s what I did with my edit of AOTC and I think it turned out well.
If you can help with VFX, that would be great! I know of a few guys who do great Vader impressions - namely the guy from Shards of the Past - I’m going to give him a shout. I jsut didn’t want to dive too deep without knowing if i could make the VFX happen.