Vice Presidential selections definitely matter, which is something we saw recently when John McCain made a poor selection. The selection of Sarah Palin dragged down McCain's campaign quite a bit. We also saw the selection of Al Gore help Clinton's campaign, as it helped resent a unified brand to the electorate.
Too often in the past 32 years we've seen the wrong vice presidential candidates drag down the presidential contender in the polls and in the voting booths. Remember Geraldine Ferraro, the country just was not ready for a woman to be vice president. Considering Hillary's bid, you have to seriously weigh in if she's careless with email will she be careless with security and be a victim leaving behind a less than suitable Vice President to lead the country.
Vice Presidential picks rarely cause poll numbers to go up, but that can easily cause poll numbers to go down. Case in point would be Sarah Palin in 2008 when she single-handedly made an entire country question John McCain's judgment. The key to the significance to a VP pick is "Do no harm."
Selection of vice president candidates has no bearing election outcomes. The proof essentially is born out by vice president debates, which are among the most low rated TV broadcast events, probably even lower rated then MS NBC and CNN. Nobody cares about the vice president, Unless it to Donald Trump. Then the press can't wait to disparage the pic. Just watch when Hillary selects her vice president candidate. No one will care. God help us all if this woman is elected.