Also, there was no way Vader could have known for sure he didn't cause Padme's death. The person he trusts most has told him it was his fault, and he probably has no way of rationalizing it any other way, especially after her funeral. So Vader thinks the baby died with her until he encounters a very intriguing rebel. I think he had an inkling Luke was his son right then, and confirmed it later when he finds out Luke's very obvious last name. As to why Yoda and Obi-wan didn't change his name when they whisked him into hiding is a little odd, but they knew no one would be looking for him when then, and he just had to be out of the way.
My guess is Obi and Yoda hoped to have him fully trained before he ever met Vader, and by that point there would be no reason to hide him.