Dear Ismail Hossain,
Are you from HR Department? If yes, then why do you find problem in identifying the vacancies that need to be filled is not understood.
Vacancy arise on three counts. One is when regular employee resigns. In that case whenever you receive the approved resignation letter, the arrangement of replacement should begin immediately. Second is when regular employee absconds. Anyway, that you will come to know through attendance records. You may wait for few days/weeks and if the employee is not turning up then you need to arrange replacement. Lastly, when the new position is created. In that case you could be intimated officially by the approving authority whenever a new position is created. In that case, you need to do the recruitment.
Hope the above explanation satisfies you. If your query is still different then please confirm.
Thanks,
Dinesh Divekar