This is how I understand those three.
Occupation - it's a proffesion. Type of job.
By calling I understand that someone is passionate about something and that's what he/she want's to do or become in life i.e. She likes being around children, she wants to become a teacher. That is her calling.
Vocation for me is a particular occupation for which you are trained.
Correct me if I'm wrong.
Student or Learner