From what we learned in episode 3 Anakin believes that he was the killer of Padm'e, but when he supposedly killed Padm'e she was still pregnant. Then in episode 5 Palpatine tells Vader that Luke is his son. This should send a big red flag in Vader's head. If Luke was his son then he couldn't have killed Padm'e, and she had to have died after giving birth. So in conclusion Palpatine is dumb for telling Vader and Vader is dumber for not catching on.