It is conventional wisdom in American elections, that vice-presidential candidates are rarely pivotal, and then only that a poor candidate (as with Sarah Palin) might detract from a campaign.
The most commonly cited exception would be 1960 when Lyndon Johnson helped Kennedy carry Texas, but even in that case his home state alone wouldn't have altered the result.
These observations are applicable in typical election years, but there has been little about 2016 that has been typical. The personalities and strategies of the presidential nominees themselves are even more likely to eclipse their running mates this year.
Of the two parties, the Democrats with Hillary Clinton are more inclined to undertake conventional considerations in selecting their vice presidential candidate.