Oh for the good old days (not that long ago actually) when the choice of the vice-presidential candidate was a hasty decision arrived at usually on the last day of the party's convention! The first one I can remember well was JFK's somewhat surprising choice of LBJ the night Kennedy was actually nominated. While Kennedy had arrived in Los Angeles as the frontrunner, it wasn't until the actual roll call of the states that he secured the nomination for certain. That night, after the roll call and Kennedy's unprecedented visit to the convention, he and his advisers settled on the choice of running mate. It was a surprise to many at the time and was controversial among some more liberal elements in the party (who had already had to swallow Kennedy for president, a less liberal candidate than some others), In retrospect, of course, it was an obvious choice and was probably crucial to Kennedy's winning the election - one of the few times the VP candidate really made that much of a difference.
Four years later, President Lyndon Johnson used the choice of his running mate to create the only real suspense at the Atlantic City convention. He too made an impromptu appearance the night of the roll call to announce his choice of Hubert Humphrey, who was duly nominated the next day.
Then there was the catastrophic 1972 convention, After McGovern had successfully won the nomination, he selected Senator Thomas Eagleton as his running mate. but that was the year the left wing of the party had completely taken over the convention, and so the country was treated to a shamefully narcissistic spectacle of delegates nominating all sorts of frivolous VP candidates just to make their point, with the result that by the time Mcgovern got to make his acceptance speech it was the middle of the night and much of his audience had already gone to bed. I was at a party that night at which we were supposed to have steak at the moment Mcgovern stepped up to the rostrum. We stuck to the schedule, even though that meant having the main course much later int he night than anyone had anticipated!
It was Ronald Reagan who created the new custom of announcing a running mate before the convention, when he did so in 1976 in an unsuccessful attempt to help his candidacy against incumbent President Ford. Since then, however, as the nomination has increasingly been secured somewhat well before the convention, candidates have added excitement to the runup to the convention by making a big show out of introducing their candidates in advance.
Of course, it is a good thing if candidates take more time than they used to in making this choice. In the last 100 years, three vice presidents have succeeded to the presidency on the death of the president. (A fourth, Gerald Ford, likewise succeeded when Nixon resigned, but Ford had not been nominated at a convention or elected as a running mate.) All too often, however, the VP choice has become yet another media-driven circus. Some running mates of excellent quality have been picked in recent elections (e.g., Al Gore, Joe Biden) but the increased interest in the VP spot has also resulted in disastrous choices designed mainly to make a media splash most obviously McCain's choice of Sarah Palin in 2008.
It is unlikely that either candidate's choice of running mate will swing this election, an election in which the electorate already has strong impression of and opinions about the candidates at the top of the ticket. So the media-generated interest in the selection is largely that - a media phenomenon. It sounded downright comic to me yesterday when the news switched from the UK's getting a new prime minister to the gossip about Trump's and Clinton's possible running mates - as if the two were comparable choices.
Historically, most candidates might have done just as well or better without a running mate. The whole process is a reminder of what a peculiar contrivance the office of Vice President really is in our system - something the very first holder of that office, John Adams, so very famously noted when he complained to his wife Abigail, "My country has in its wisdom contrived for me the most insignificant office that ever the invention of man contrived or his imagination conceived."