We only have random rosters with up to five days of duty in a row. Vacation days are based on a week of 7 days of work, which means that i have to take 7 days of vacation for 7 days. And that is the shortest possible vacation period anyway. However, for any vacation period of seven days or more we do get one fixed off day before and after the vacation, out of the normal monthly off days (11 on average). That is one reason why we get 42 days of vacation a year, not the usual 30 our office staff (with 5 days of work per week) gets.
Since we can request 4 days off work each month a cleverly planned vacation of 7 days can net up to 17 days off work.