In ANH Obi Wan says to Luke 'Your father wanted you to have this when you were old enough, but your uncle wouldn't allow it.' which suggest Anakin new about Luke before he becomes Darth Vader. So Vader knows about Lukes existance from the beginning right?
He also told Luke that Darth Vader betrayed and murdered his father. So I'd take anything that crazy old wizard said with a pinch of salt if I were you.
Ben doesn't sound like he was giving false information, that part of the seen comes across as genuine.
Maybe from obi's knowledge of his padawan what he ment is "Your Father [would have] wanted you to have this when you got older"