besides, if you have anakin saying that he wants his son to have this when hes old enough and you have ben saying to luke that your father wanted you to hae this when you were old enough, you tip the audience off as to vader being lukes father. Thus ruining the surprise of vader himself telling luke he is his father. I can't see any way of doing the prequels without ruining that surprise in ESB. The only answer is don't even do the prequels at all