My impression with post-RotS Lucas comments is that Vader has no idea that Luke is his son, until the Emperor tells him in the asteroid field. But, hey, this is from the guy who is on record as saying the dark side is more powerful than the light, and that Han would wait for Greedo to shoot before firing himself. 8-l
This does weaken Vader's character immensely, so I choose the story that Vader has known for quite some time (how long is unknown), and is following his own agenda regarding Luke behind the Emperor's back. Vader gets a thoughtful, ruthless quality that is not there if he is actually unaware of Luke's origin.