Alan is correct, the question depends on context. If someone asks your occupation, you would be repeating the question as if surprised implying either a reluctance to answer, or surprise that the person is interested in you. Alternatively, you may have a very difficult or embarrassing choice to make and have great difficulty in deciding therefore you could ask (or think) 'what do i do ?'

Someone is asking what you do for a living. What is your vocation?

Can you give a sample sentence or two. Because it's meaning depends on the context.