It's an M. Night Shaymalan movie, don't try to make sense of the "plot".
They spent what, a week at most with Marty/Calvin Klein? Yeah, 5 November 1955 to 12 November 1955. Do you remember everyone you spent a week with 30 years ago?
http://en.wikipedia.org/wiki/Autopilot
Vader wasn't aware his children survived the death of their mother. I can't even recall if he was aware that she was having twins. Vader had an entire galaxy to watch over, never mind one kid from one planet with his name that he wasn't even aware existed. So, it's not inconceivable that he wouldn't find Luke. Besides, only a handful of people knew that Vader was Skywalker, and I would assume that they were all running interference to make sure that Luke's name/heritage was not broadcast to the galaxy.
They have advanced solar/lunar positioning, so their feeding cycles adjust to local time? Eh, this one I just ignore. They're fun movies and Phoebe Cates is hot.