As this tweet demonstrates, should never be considered an authoritative source for Real World information—While it is true that Speaker Pelosi is next in line after the Vice President in the order of succession, Section 2 of the 25th Amendment specifies that the President (i.e., President Pence if Donald Trump were removed from office) “shall nominate a Vice President who shall take office upon confirmation by a majority vote of both Houses of Congress.” Note the use of the word shall. President Pence would be required to nominate a new Vice President subject to the approval of both houses of Congress. When the new VP was sworn in, the Speaker would move back to her proper place in line.
Section 2 has been triggered twice. When Vice President Agnew resigned, President Nixon nominated Gerald Ford to replace him. Congress confirmed that nomination. When President Nixon resigned and Gerald Ford became President, President Ford nominated Nelson Rockefeller as VP, and Congress confirmed him.
Perhaps the kids over at Vox think that Mike Pence couldn’t think of a sufficiently non-controversial nominee for VP and that the office would remain vacant until after the next election. Or maybe they’re too young and too ignorant of History to remember or know what happened 45 years ago.