Good point. I was trying to look up what the procedure was for replacing a VP before that, but failed. It almost must have been done at least once before then right? I couldn't recall or find though.

There was, in fact, no procedure for replacing the VP; the move for the amendment to correct that came about after, and motivated largely by, the Kennedy assassination and the vacancy in the term Johnson completed; historically, vacancies in the Vice Presidency hadn't been considered important enough to do something about, but the Cold War changed that.