What would happen if Lincoln survived his assassination but was left brain dead from it.
Article II, section 1 of the Constitution provides, "In Case of the Removal of the President from Office, or of his Death, Resignation, or Inability to discharge the Powers and Duties of the said Office, the Same shall devolve on the Vice President..."
So the only question is whether Andrew Johnson is President or Acting President (as with Tyler, it turns on whether "the same" refers to the office or to its powers and duties). Assuming that Lincoln is beyond recovery, does this matter? Surprisingly, the answer may be Yes. As I explain elsewhere,
"Yet recognizing Johnson as President set an obstacle to conviction on the most serious charge--that Johnson had violated the Tenure of Office Act by removing Secretary of War Stanton, a Lincoln appointee. The Act provided that an officer appointed with senatorial consent could be removed only with such consent. However, the Act also provided that an appointee's title to office expired one month after the end of the term of the President by whom the officer had been appointed. So that if Johnson was still the Vice President, he had removed Lincoln's appointee during Lincoln's term of office--and had therefore violated the Act. But if Johnson were truly President, his defenders argued, Stanton's title to office had expired a month after Lincoln's death, and therefore Johnson's removal of him did not violate the Act... In any event, Senator Fessenden, one of the Republicans who voted for acquittal, said that he did so precisely because he believed Johnson was President."
https://groups.google.com/d/msg/soc.history.what-if/ltV0Yl6EmJw/Zw3FoOQqgJ8J