So one of the complaints about the Saga is how someone named Luke Skywalker was able to remain hidden on Tatooine because of his name, that his father was born there, etc. I just attributed it to force fitting a story. But now it kinda makes sense. Obi-Wan brings Luke to Beru and Owen and probably explaining that this is Anakin and Padme's child. He probably didn't provide any information regarding other than he was an orphan. Going by that, it's not a far reach to give him the name Skywalker as they were probably unaware that Anakin is alive as Vader. Secondly, the odds that Vader would ever come back to Tatooine which was established as being far from the core worlds and ruled by the Hutts, was slim. Plus, there would be no reason for Vader to ever return to Tatooine prior to the events of A New Hope.