Hear me out.

Do you think when Vader said to Palpatine “He’s just a boy” he said it to try and protect Luke? Think about it, he knew padme was pregnant but thought the children probably died when she did, but now, the remanent of Padme exists, Luke. Vader must’ve realized this and immediately knew he couldn’t kill his son, so he tried to defend him by saying he would turn Luke to the dark side.