Each subscriber is allowed to have 36 vacation days per year. These days can all be used at one time, or in any manner that the member wishes to use them.
There is no way for one member to see the number of days another member has remaining or how many days they have currently in use. The vacation icon is visible by their name while they are on vacation status.
Days that are not used are lost at the end of the year, when the total available is reset to 36. (In other words you cannot add unused days from one year to your total available days for the next year.)
I hope that answers your question.