That is one of the most ridiculous. If you watch the original cut, it is clear that Vader knows who Luke is. That is never in question. Anakin knew that Padme was pregnant so knowing her baby survived should not come as a complete surprise. And it would be easy to make the connection of Luke Skywalker (A name any good spy could have ferreted out) from Tatooine (and again, a good spy could ferret out he was raised by Owen and Beru Lars outside Anchorhead). So it would be easy to put the pieces together. Vader seems to have all those pieces in TESB and in the original cut he plays dumb to Palpatine and just suggest turning Luke. The altered dialog in the SE serves no point except to point out how little Lucas has paid attention to his own films and their continuity over the years. The addition of these lines was not needed and really makes no sense as we already know that Vader knows he is after Skywalker so that bit which seems to be intended to convey to the audience that Palapatine is revealing that Padme’s child survived, but earlier in the movie we already heard him talk about Skywalker and we know he knows what that name means. So it is a really stupid change.