Thanks for the help everyone. However, since I don't rely on EU stuff for my answers, there has to be some sort of indication in the movies how suddenly Vader knows who Luke is.

I've heard that Lucas want to film (for the Archival Editions) a scene for Episode IV where Vader and the Emperor are talking after the destructin of the Death Star and Luke is mentioned or at least hinted about. If that were done, it could go a long way to fixing that little plot hole. It just seems strange (leaving all of the EU books, and comics out of the equation) that suddenly out of nowhere, for Vader to know who Luke is and is now obsessed with finding him. Also, in Empire, Vader knows that Obi-Won was helping Luke. I know that it's a minor plot point, but it has always bugged me.