Fair enough. I had a really long response but it got deleted, so I’ll just summarize. If he knew a single one of Vader’s accomplishments (rumors of them were definitely spread by the witnesses), he would’ve never believed that. Vader served the important purpose of striking fear into those that would oppose the Empire, keeping most of the galaxy in line. He probably served as an officer in the Republic as well, possibly alongside the Jedi. Even if he didn’t, he’s in a room full of people that did. It’s ridiculously stupid to insult your extremely violent boss, especially about something that he’s provably correct about.