Passengers can book vacant seats after chart preparation by visiting the nearest railway reservation counter or using the IRCTC website/app. To book a vacant seat, passengers must first check the availability of vacant seats on the desired train using the Train Enquiry feature or the IRCTC website/app. Once the availability is confirmed, passengers can go to the reservation counter and fill out the reservation form with the required details, such as the train number, date of travel, class of travel, etc. If booking online, passengers can simply log in to the IRCTC website/app, select the train, and enter the required details to complete the booking process. It is important to note that booking vacant seats is subject to availability and not guaranteed.