IN THE VANDER WAALS EQUATION,the focus is on real gases ....right.......so taking into consideration the flaws in the ideal gas kinetic theory , we consider the additional pressure of the molecules when they strike themselves n the walls of the container........in case of volume, the volume occupied by the gas is less as compared to the ideal gas volume since we consider molecules to be of negligible size compared to the whole volume.........so in case of real gas we deduct the total volume of the molecules n add the extra pressure from the molecular collisions