Big news sites will do things like write up a story announcing a presidential win for each of the candidates, then only publish the one that matters after the election. This way they can have a story on the front page within minutes (seconds these days) of having official results.
I wouldn’t be surprised if Vox had 1 of these lined up for each of the likely VP picks. If not that, it certainly wouldn’t be unusual for a journalist to do their homework on all of the candidates and to have the rough outline and some key facts ready to go for each. If you’ve already done most of the research, assembling the final story shouldn’t take more than a couple of hours.