There's no way Vader could have hidden his pursuit from Palps ... and indeed, it was never suggested that Palps was unaware of Vader's obsession with Skywalker until these new DVDs (the previous suggestion was that Palps hadn't felt any serious disturbances in the force until Skywalker pursued Jedi training). So now you've got Vader saying, "How is that possible?"when told it's Anakin skywalker's son, even though the opening crawl establishes that he had thousands of probes and Lord-knows-how-many ships looking for Skywalker?
Either Vader is so stupid that he hasn't put two-and-two together, or Vader's so stupid he thinks the Emperor is too stupid to put two-and-two together. No one was this stupid before the DVDs.