Remember Biden's last debate where he just laughed hysterically like Johnny Carson at everything that was said. Limbaugh said "You witnessed the mental breakdown of a politician" or something like that. And still the people vote for him.
Nobody really votes for the Vice Presidential candidate. They are on the ballot, but people vote for the Presidential candidate. Veep nominees only have an impact on the margins, which may or may not come into play in a very close election.
It was a non factor in 2000, 2004, and 2012, In 2008 Palin probably brought in some Conservatives suspicious of McCain, but not enough.