Darth Venal said:

Well, aren't you missing the fact that Vader didn't even know he had a son, and so wouldn't be looking for him?

Vader was extremely strong with the Force. That provides a great deal of insight and intuition. He felt that Padmé was alive when he left her. There were obvious motives to lie to him. Yet he never suspected he had been lied to?

He was obsessed with the survival of his family. (At least, he was obsessed with the survival of his mother and Padmé. It’s reasonable to assume he would have the same attitude about his child.) He had the entire resources of the Galactic Empire at his disposal. Yet he was so 100% certain that he had been told the truth about his child not surviving that he never bothered to make even the most cursory investigation and have somebody check out the Lars household?

Explaining away the error by assuming that Vader was a COMPLETE FLICKING MORON is not very satisfying.