Forum:KB:ESB

In Empire Strikes Back, Darth Vader says "that's it they are there" (referring to Hoth) "and Skywalker is with them". Later when he communicates with the Emperor he (the Emperor) says Skywalker is the son of Anakin Skywalker and Vader says "can this be true". My question is. Vader knew at the beginning of the movie that Luke was a Skywalker so surely he knew the he was related to him, surely?