[DuncanWaring]

Since the assassination of William McKinley, when Theodore Roosevelt took over as the nation's youngest president, seven former vice presidents have either by election, assassination, succession - ... - taken the oath of office to "preserve, protect and defend the Constitution of the United States of America".

Why no reference to Abraham Lincoln/Andrew Johnson?

Journalistic incompetence?

[ozzymandus]

Possibly because Lincoln came before McKinley, and the writer says “since McKinley”? There were several other VP’s who became President before Teddy, too.

[null and void]

Lincoln was a republican. His assassination doesn’t count.

[Stonewall Jackson]

Here's a list of the Vice-Presidents who went on to become President.

John Adams, Thomas Jefferson, Martin Van Buren, John Tyler, Millard Fillmore, Andrew Johnson, Chester A. Arthur, Theodore Roosevelt, Calvin Coolidge, Harry S Truman, Richard Nixon, Lyndon B. Johnson, Gerald Ford, and George H.W. Bush.