You don't enter vacant days anywhere, as they have absolutely no impact what-so-ever on your tax liability. The only things that matter are days rented and days of personal use while the property was classified as a rental.
Vacant days do not count against you for anything. They are still "business use" days if you did not live in the property as your primary residence, 2nd home, vacation home, or any other type of personal use while/when it was vacant and classified as a rental.
