The press couldn't "vet" if their lives depended on it. They barely even report, they just yell a lot about the same things over and over.
It is also about controlling the media with scandals. When Biden ran for president for one year, the media covered him and his history, so by the time he became VP, any scandals were "old news" and not worth reporting.
But when Palin became VP, anything controversial from her past was released on a daily basis in the news, creating this mountain of scandal, that may have been contained if she were in front of the national press for 12 months. Even the smallest scandal would be blown up in that 2 month time frame.
So the vetting is part of the media game - do you pick someone completely new like Rubio or Portman and throw to the wolves for 3 months and have his life dissected? Or do you pick someone who has been in the national spotlight for a decade and has built relationships with the media?
I just think Romney will pick a well-known name and not a fresh face.