WOW! I got a bigger response than i expected on my previous blog 'A Lucas Error?' (link), lol. Some of you have great ideas! I've been extremely impressed. A few things and opinions i'd like to add:
1) in the opening crawl it says the Vader is chasing 'Skywalker' throughout the galaxy. The fact that it says Vader is chasing 'Skywalker' doesn't necessarily imply that Vader knows who Luke is. We the audience are being informed of this chase. It's like saying Spain is tracking a spy. We the audience know who the spy is so we're told that Spain is chasing Iran but obviously Spain doesn't know that Iran is the spy. I know that doesn't really matter because Vader does know that it's Skywalker but i just wanted to point that out, lol.

2) Someone said that the Emperor knew about Luke and Leia the whole time. My only question is, if that's true, wouldn't he have secretly had them killed knowing that they could be a threat to him and Vader, as Obi Wan informs us in ROTJ? Why take any chances?

3) Throughout the Star Wars series, those force sensitive are able to sense other force sensitives. However, they usually are not aware of who they're sensing unless they've been in contact with them before. Knowing who they are and identifying their force signature. For Example: Anakin sensing Dooku on the Invisible Hand in ROTS. He came in contact with him on Geonosis and made himself conscious of the feelings that he felt in relation with Dooku and recognized those feelings while on Greivous' ship. While on the Death Star, Vader is able to sense Obi Wan, having been in his presence before, but not Luke. It doesn't necessarily mean he doesn't sense him but he doen't know who he's sensing. Luke's force identity hasn't entered into Vader's memory quite yet. Eventually, sometime after A New Hope he understands his force feelings of who Luke is, but not as his son. As has been said, Vader believes his child died with his wife. Now suppose that Skywalker is a very common name. Under these conclusions, Vader has no reason to believe that Luke is his son. There are many Skywalkers out there and Vader doesn't believe he has a son, so under these pretenses Vader knows he's chasing a Skywalker but he doesn't know it's his child. Only until he's informed of his identity by the Emperor does he become privy to this info. There's always the argument that Anakin felt Luke's presence and identified this feeling as his son while in Padme's womb. However, if this is the case wouldn't he have felt Leia as well? Perhaps Luke was more force sensitive than Leia while inside of their mother. Although, Leia being the product of someone who was the God-like son of the force itself would more than likely be, to some degree, force sensitive herself. That being the case Anakin should have felt another being inside his wife and if so, wouldn't he have informed Padme of his discovery? Under these conditions i find it hard to believe that Anakin felt his children before their birth. With this logic, Vader was chasing a Skywalker who coincidentally was his son. He was genuinely suprised to hear from Palpatine that Luke was indeed his product of offspring. Then again, he could have been like Leia and 'known' but not known, if that makes sense, lol (This theory is under the impression that having visions, ie Luke seeing Han and Leia, and sensing someone's immediate presence are two different experiences)

4) And finally, when it ultimately comes to the question of whether Lucas did infact make a mistake, I believe he might have. There are many complex explanations as to why Vader was chasing a Skywalker that he, perhaps, didn't know was his son. In the end, I once read that Lucas was afraid that the Star Wars movies might be too convoluted for fans so he tried to keep his story as simple as he could. It seems to me that he could have easily explained in the movie what was going on but didn't. I don't know whether to think that Lucas did infact make a mistake, and if he held true to what I read it would seem that he did. Honestly, I like to think that he did know what he was doing. It makes it that much more fun to theorize about what he might have been thinking. The guy's a genius!

Feel free to check out my other blog: http://blogs.starwars.com/blatervog/1