Is being alive a qualification to be elected? I have no doubt Trump's people will try to pull an Air Bud "there's no rule that says a dog can't play basketball" move.

A bigger question is, how could we tell that even happened?

Edit: I mean short of him dying or becoming a drooling vegetable. The man is already deeply deranged and exhibiting significant mental declind

Once the individual state filing deadlines have passed, it's likely that Trump/Vance would have to remain on the ballot. But this is one instance where the Electoral College helps, because the election is not for the candidates themselves, it is for electors who support the ticket. And the final certification of electors is up to each State Legislature.

So, if God exercises His Ultimate Veto on Trump's Presidency, it will be commonly understood that Vance would take over the ticket. He will pick a new VP, and any former Trump/Vance electors would likely cast electoral votes for Vance/Chesterfield instead. While it is possible that State Democrats might challenge this, it's also absurd to force electors to vote for someone who can no longer take office, so it's likely that States would give their blessing to the substitution. Particularly if Harris is the overall winner regardless, and the Republican EC votes are simply for the historical record, and have no effect on the outcome.

His cult would still vote for him if he's got a pulse.

Based on what I've seen Republicans claiming online, prompted by recent changes in the Democratic ticket, such a change would be very dangerous to our democracy and therefore we should go forward with Trump being President despite the minor setback of being dead.

after the Yub Nub festival, presumably Vance should step up, but, given his shocking unpopularity, the RNC could nominate someone entirely different.

nobody knows. it has never happened before, so it would be new territory.