Vader, for all his wording to Luke and Padmè didn't actually want to rule - he wanted to enforce their wills with a little of his own mass murder of slavers. So while he doesn't know everything going on inside his grandson's head, he understands better than he realizes.

"It's heavy leading. Leading my men for so long, I learned what I would and wouldn't accept from myself. What I wanted to be. Trying to step into this," what he was now, "without that would be setting myself up to fail. You're going to need help, and need to study leaders from the past. Not just me."