For all you courses you must register through VUnet. This way, you find out immediately if a place is available. All activities for which you are registered will be displayed in your personal timetable, which also includes any timetable changes. If you have not registered for a course then you will not be admitted to that course, you will not be assigned to a group, you will not be able to use Blackboard, you will have no timetable, your grades will not be recorded, etc. In short, you will not be able to take part.
From the moment that you are conditionally registered for a programme, you can sign up for specific courses via VUnet.