John has a real vocation for being a doctor - John is a doctor and this is his passion. John has a real vocation to be a doctor - John is not a doctor yet but he is passionate about becoming one.

No. This relatively rare meaning of ‘vocation’ needs ‘for’. ‘Vocation’ here refers to a belief or passion. You can also just say (more commonly) ‘John’s vocation is medicine.’

I seldom use this meaning of "vocation". Perhaps the British use it more. I would say "John is a doctor by vocation". However even though I seldom use it, the other meaning as a "calling" makes perfect sense. That's what its Latin roots mean. Another good word to keep in mind is "occupation". It is more flexible than most people realize. It can mean a career or a job but it can also just be a hobby to which you devote most of your time. Literally, it just means the thing that occupies you.