Great points Shem. Your Argument for Vader not knowing Luke exist is solid. I guess I am now speculating on more of what that line meant then. I thought maybe there was a chance that he had recently felt another disturbance in the force like he did at the beginning of the game with Kento and there was someone more powerful than starkiller ....................maybe it was luke and he didn't know it was actually Luke yet.
I don't know, just really curious as to what that line meant.