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You say that I have not cited "any regs, canon law or conclave law." Yet, I wrote a very detailed post doing what you say I did not do. Here is the link to that post, in case you missed it. But here is the relevant part, again, to your question about the trigger for "a vacancy in the Apostolic See." I said the following:

3. How does the Apostolic See become vacant? Does it become vacant on the "resignation" of the Roman Pontiff. No, as UDG 14 and Pastor Bonus 6 state, the Curial offices of the Apostolic See only become vacant upon the death of the Roman Pontiff:

UDG 14. According to the provisions of Article 6 of the Apostolic Constitution Pastor Bonus, at the death of the Pope all the heads of the Dicasteries of the Roman Curia — the Cardinal Secretary of State and the Cardinal Prefects, the Archbishop Presidents, together with the members of those Dicasteries — cease to exercise their office.

PB Art. 6 — On the death of the Supreme Pontiff, all moderators and members of the dicasteries cease from their office. The camerlengo of the Roman Church and the major penitentiary are excepted, who expedite ordinary business and refer to the College of Cardinals those things which would have been referred to the Supreme Pontiff.

Notice that there is no mention that upon a "resignation," the Curial offices "cease from their office." No, only upon "the death of the Pope" do those offices become vacant.

Here, I quoted two Apostolic Constitutions (Universi Dominici Gregis and Pastor Bonus). Both of them explain that only upon "the death of the Pope" will the "heads of the Discasteries of the Roman Curia- Cardinal Secretary of State and Cardinal Prefects..." cease "to exercise their office." These Curial Officials mentioned cannot "exercise their office" if the office is vacant, right? It would no longer be "their office" if it was vacant. Therefore, a papal resignation, which obviously can happen, does not trigger the loss of office (or "vacancy") in the offices of those Curial Officials. Only the "death of the Pope" triggers the vacancy in those Curial Offices.

And, as I stated in the same post, Canon 861 (1983 Code) defines the offices that legally make up the Apostolic See:

Canon 361 In this Code, the term Apostolic See or Holy See refers not only to the Roman Pontiff but also to the Secretariat of State, the Council for the Public Affairs of the Church, and other institutes of the Roman Curia, unless it is otherwise apparent from the nature of the matter or the context of the words.

So, when the Pope resigns, the Apostolic See is not yet vacant. It only become "lawfully vacant" upon "the death of the Pope." And a papal election cannot take place until AFTER the Apostolic See is "lawfully vacant." What part of what I have cited is still not clear to you?