No one votes for the VP. That pick is nothing more than a shore to the base. The left likes to rant and rave about how they could never vote for Palin but in reality none of them would have voted for McCain anyway. Those were never votes to lose as they were never votes for the republicans in the first place.
Polling from 20008 shows you to be quite wrong. When McCain first picked Palin and announced it at the Nutter Center (how fitting), Wright State University, Dayton, OH, his national poll numbers shot up on par with Obama, for about 1week.
And then Palin opened her mouth for interviews, and the slide began for McCain, before the economic meltdown created by Bush 43 began.
In later polling, asked if Palin would be competent to assume the role of the Presidency, if needed, the resounding answer was NO.
A good VP pick may be overlooked, but a bad VP pick can kill a campaign. See: McGovern/Eagleton 1972, Mondale/Ferraro 1984. And now: McCain / Palin 2008.
So, yes, it made a difference.