This chart illustrates that vice presidential debate viewership, as a percentage of the total US population, has varied between 10 and 25 percent since 1976. The notable exceptions: 1984, when Democrat Geraldine Ferraro debated Republican Vice President George Bush (24%); 2008, when Republican Sarah Palin debated Democrat Joe Biden (23%); and 1976, when Democrat Walter Mondale debated Republican Bob Dole (20%). The least watched: Gore-Kemp in 1996 (10%).
The viewership data are from Nielsen and do not include PBS station estimates. The "percent population" is my measurement, not one used by the TV industry. It is simply Nielsen data divided by population using these population statistics.