According to the National Archives, it seems like succession might not necessarily apply and would likely be argued over by lawyers should he die before the Electoral College meets. I'll paste their text below, and I pasted the link in a separate answer
What happens if a candidate dies or becomes incapacitated?
There is no Federally-required process to follow if a candidate who is projected to receive electoral votes dies or becomes incapacitated between the general election and the meeting of electors. However, individual States may have their own requirements that govern how electors must vote at the meeting of the electors. In 1872, when Horace Greeley passed away between Election Day and the meeting of electors, the electors who were slated to vote for Greeley voted for various candidates, including Greeley. The votes cast for Greeley were not counted due to a House resolution passed regarding the matter. See the full Electoral College vote counts for President and Vice President in the 1872 election.
We don’t know what would happen if a candidate who, dies after or becomes incapacitated between the meeting of electors and the counting of electoral votes in Congress.
The Constitution is silent on whether this candidate meets the definition of “President elect” or “Vice President elect.” If the candidate with a majority of the electoral votes is considered “President elect” before the counting of electoral votes in Congress, §3 of the 20th Amendment applies. That section states that the Vice President elect will become President if the President-elect dies or becomes incapacitated.
If a winning Presidential candidate dies or becomes incapacitated between the counting of electoral votes in the Congress and the inauguration, the Vice President-elect becomes President, according to §3 of the 20th Amendment.
I think in the constitutional sense, "President-elect" and "Vice President-elect" refers to the candidates after the electoral college votes and before they take office. The constitution doesn't take into account popular votes, as presidential electors aren't even chosen by popular vote in early days.
I'm not sure how there's much of any argument for it. It's pretty clear that a person only officially becomes vice/president-elect after being voted for in the EC.
Though what happens it the EC elects a dead man? Many states REQUIRE their electors to vote as told. Probably would see more state level law crises at that point. Resolutions to make the electors vote for the VP candidate if the presidential one dies.
i remember looking that stuff up before... iirc, vance would not enter the line of succession until the congressional count of electoral votes in early january.
Which again, is inevitable. Good luck getting the Supreme Court to rule against the Republicans if this happened. The ticket was on the ballot, not the individual candidate for President.