Are you sure about this Katana? There was a time when the President and Vice President were elected separately. So you could have a President from one party and a vice president from another party. I had always thought Andrew Johnson was a democrat elected under that old system.
I don't remember precisely what year the change in elections came, so you could be correct. However many younger people assume that our current system has always existed. It is easy to make a mistake and think the president and vice president were always had to be from the same party.
Under the old system that was not the case.
You are correct that at one time candidates from two parties could (and in fact before the advent of formal parties invariably were) the first and second finalists coming out of the Electoral College (hence Thomas Jefferson being John Adams VP). But I believe that was changed very early on since it became very obvious that the death of a president and his replacement by someone of an opposite political persuasion would cause havoc. Imagine the conspiracies that would be going on if John Kerry were Dubya's vice president.
In any case you and the author are correct that Andrew Johnson was a democrat.