Vader didnt know specifically about luke in ANH, where as in ESB his sole purpose was to draw out skywalker and convert him to the dark side, see his explinaiton of it in RotJ. That was why he was torturing han, just to send out the signals to lure skywalker in. Who cares if he keeps one person alive if it means a nice new apprintice with which he can "rule the galaxy with as father and son"? I am sure leia would have been delt with after luke was at his fathers side.

if the ship is landed and therefore not on the radar, which why would you be looking at the radar if you where flying into your home boys city?, and on the other side of the city. Remember that han is escorted to a specific landing site, and that he is flying through the clouds both which would have both effected his vision...