Some older Volkswagen buses may have seatbelts, but these are often lap belts, which are not designed to restrain a child in a car seat safely.
You should only attempt to install a car seat in an older Volkswagen bus if you have confirmed that the seat can be properly secured and that the vehicle has modern, three-point seatbelts for use with the car seat. You should also consult with a child passenger safety expert or the vehicle manufacturer to determine if the car seat is safe to use.