There is no single more important qualification, from the standpoint of the person who picks the vice presidential nominee, than trust. This is what John McCain's veepstakes czar will surely advise him, lest he (she?) be guilty of malpractice.
And John McCain, although he brings people in and out of his inner circle with surprising ease, doesn't trust all but a handful of people, usually folks who have suffered along with him. The presidential race doesn't cut it. No amount of assurances, cajoling, convincing, pressuring and persuading can build a level of trust between Mitt Romney and John McCain.