Stampede said:
A grim sense of humour. I always enjoy the part where Vader reminds Piett of how he reached his station. "Dont fail me again...admiral."
I actually think that line is not really representative of his twisted humor. I think he was dead serious when he said that- and intended it as a threat.
The line I always loved for its twistedness was (after strangling him) "Apology accepted, Captain Needa."