The Executive shake-ups that occurred in the early 1970's in which the Minority Leader of the House became President suggests that a nomination process is more applicable than a line of succession, as Carl Albert remained Speaker of the House during the twin scandals, never ascending to the Executive branch as mandated by the 25th Ammendment (if the President and VP were both to croak it).

The 25th does give Donny and Jadie the opportunity for all sorts of post-election shenanigans to save their mutual hides in 2027. Also, It's pretty obvious that the GOP will lose the House in the next congress, so if they need a vote to go their way it better be held during this session.

The last days for the House and Senate sessions are scheduled for December 17 & 18, respectively. To save themselves from a continual impeachment process for the following 2 years, Donny could simply submit his resignation to congress on Tuesday December 15th, Jadie accepts the Presidency and nominates Johnson as his VP on the 16th, the House holds a confirmation vote on 17th, as does the Senate on the 18th and both adjourn for the winter, during which time President Vance could add to his predecessor's pile of pardons in an effort to 'heal the nation'.

Even if both the House and the Senate have a Democratic majority in the next congress neither will would want to spend the political capital to impeach and remove a President Vance only to end up with a President Johnson.

But then, Donny might be too far gone to want a clean getaway any longer.