Hmm, I'm not sure about the "caring for Luke's well being" part. Seems to me that he wants a disciple for himself and if he brings Luke in too soon, he'd likely end up as Palpatine's disciple. Vader needed the chance to get Luke on his own to play the "I'm your father" card.
Let the Lars' take care of the toilet training. He'll remember him when he's old enough to be useful.