Replacing Vance now would also undermine any argument they might make about switching the names on ballots for the Democrats. Those aren't valid, anyway--Biden wasn't the official nominee until the convention says so; nothing was locked in--but they would now lose the possibility of using it rhetorically.
They don't, but it also has to sink in with the general public. When both sides are being switched out, there's a very obvious argument against the claim.
There is a legal point where states won't accept changes to the ballots. Also a practical one where there isn't time to print new ones. Neither point has passed, of course, but Trump was trying to imply it was.
If Biden died on November 4th, he'd remain on the ballot the next day and - if he won the majority of the EC - Harris would be the presumptive President Elect on November 6th. If Vance resigned as VP, we'd see the same results under a Trump win. We have a clear order of succession and mechanisms for appointing replacements to downticket slots like VP per the 25th amendment.
The absolute worst case scenario is that Trump would force Vance to resign and then be stuck with a House/Senate picking his VP for him. But since he's not even the President-Elect at the moment I don't think that's actually an issue.