Walz has been known to be on the shortlist for at least a week, probably longer. They probably had articles like this in the pipeline for all the likely candidates.
The campaign worked with news outlets and let them write the articles, under what's often called an embargo. Basically, in exchange for holding the story until it's announced, the journalists can write the story ahead of time.
Source: dated a girl who worked in PR and a girl who was in media relations for a large company
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.
Well, he didn't just crawl out of a hole, he has a record. The article is making the claim that he has the potential to bring together different elements of the democratic party, which ultimately is the party of everyone else that isn't voting Trump. This is a big tent with a lot of perspectives, and while democrats are largely united against Trump, that doesn't inherently mean they're just as united behind the candidate (as we just saw), and those kind of things are ripe for Republicans to pick at and promote infighting.