If VP resigns, POTUS picks VP
25 amendment
"Its next section reads: “Whenever there is a vacancy in the office of the Vice President, the President shall nominate a Vice President who shall take office upon confirmation by a majority vote of both Houses of Congress.”
Example Nixon nominated Ford when Agnew resigned.
Source: https://www.factcheck.org/2020/09/vice-presidential-vacancy-isnt-automatically-filled-by-house-speaker/