Beat the game using Maki's Vestal and the B-Vestal will be revealed.
Race the Vestal
Beat the game with every car. You can save after you beat the game with a car, but you cannot use any continues. Now you can race as Maki's six-wheeled vehicle from the end of the game.
Tsumuji
Select Attack mode and a driver of your choice, then select Tsumuji as your opponent. All you've got to do now is finish in first place and his car will become available.