My guess would be the winner's VP would be sworn in as president and I suppose that person would then choose someone to be their VP. You have to remember that a presidential election is not the election of one person but two.
The Vice-Presidential nomination often is a matter of ticket-balancing, of countering some weakness in the Presidential nominee, or is made in order to appeal to some special group of voters. It's rarely someone who was the second choice for President in the Party's nominating convention.I agree that IF Biden/Harris wins on election day and Biden dies the day after that, his Electoral delegates will vote for the ticket which makes Harris our next president.