If it is before the electoral college votes, they could just pick someone else (doesn't have to even be the vp elect)..

If it is after the EC elects the POTUS/VPOTUS (we don't elect them, the EC does), then the VP-elect would be sworn in as potus.

It's in the 20th amendment.

during the interim until inauguration of President-elect. So, the existing President's Vice President would be sworn in, to serve until the newly elected President is sworn in at the usual inauguration. (Please correct me if this is wrong.) What I'm unclear about under the above scenario, who becomes Vice-President in the interim period? Speaker of the House?

I should have been specific. What would happen if the Pres-elect died ONE DAY BEFORE the inauguration?

Would the VP-elect ascend directly to the presidency (without ever technically being the VP)?

Or does the constitution require that the VP-elect be sworn in AS the actual VP, before moving up to being the President?

Section 3. If, at the time fixed for the beginning of the term of the President, the President elect shall have died, the Vice President elect shall become President. If a President shall not have been chosen before the time fixed for the beginning of his term, or if the President elect shall have failed to qualify, then the Vice President elect shall act as President until a President shall have qualified; and the Congress may by law provide for the case wherein neither a President elect nor a Vice President elect shall have qualified, declaring who shall then act as President, or the manner in which one who is to act shall be selected, and such person shall act accordingly until a President or Vice President shall have qualified.

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president if the winner of the EC and vp (before inauguration day) meet their maker?

the house decides. It can be anyone the house choses that meets the qualifications for president. It doesn't even have to be someone who ran in the election.

That was the big fear if clinton won the election but it was close enough for a state with a red ag/soc and gov to simply refuse to certify the vote. In that case, the EC's could not be chosen by candidate and no votes for that state might be counted. It could either tilt the election or throw it to the house, which was paul ryans at that point.

Is sworn in before the President. Until that point he is the Vice President elect. Though it would be a technicality I believe they would first swear in that person to formally become the Vice President, even if such a ceremony was done in a small non-public setting in order to fulfill the letter of the law of succession.