Ziz said:
Emp: The son of Skywalker must not become a Jedi. >:-|
Vader: If he could be turned, he would become a powerful ally.
Emp: Yes...yes. He would be a great asset. ',:-)
The Emperor is smiling because Vader just gave him a cool idea he hadn't considered a minute ago.
Didn't Adywan create the second "yeeeeeeees" out of the ether anyways? That's pretty damn cool.