I think we all know and love Sidious's Speech to Anakin about Plagueis.

What do you think Sidious would say about Vader if he were in the same setting?

I would think of something along these lines:

"Darth Vader was a powerful Sith Lord. His fear and anger fueled his power, but he was also broken. His powers were torn away by self-doubt and the destruction of his body."

I just have no idea who he would be talking to at this point or what the purpose of the speech would be lol