The VP candidate has one purpose, and that is to get the Presidential candidate elected.
Sarah Palin is liked locally and scores some demographic points, but I still feel that Chuck Hagel is the rock solid favorite.
He's a "family values" conservative Republican from the "Heartland", has a distinguished military career, looks like he could still kick your ass, yet also had the guts to publicly trash the administration over the Iraq War. He's a good speaker and debater as well.
Most importantly, he brings a mainstream image that will tone done Paul's "fringe wacko" aura. People would be comfortable with him stepping into the Oval Office after Paul.
(Note, I have nothing against Sanford either, just haven't seen him in action.)