First off, I believe there is a spelling mistake.
Vocation = typically means career or occupation
Vacation = a period of suspension from work or school or a special day
I believe you would like when to use holiday and vAcation.
The meanings are very similar. I would venture to guess that they could be equally interchangable in most situations. In American however, we use them to refer to different things.
We use "holiday" to mean a special day that is excused from work/school. Christmas, Vetern's Day, Thanksgiving are all "holidays". We use "vacation" to connotate a leave from work/school. "We are going on vacation to the Rocky Mountains." "We are on summer vacation."
An easy way to think of it is 'is it short or long'? If it is only a day, use "holiday". If it is longer, such as a week or entire summer, use "vacation."