It's not. In the PBS practice something similar was implemented.

If you chose to extend you week of vacation, you checked a box and the system was going to award 3 off days before and after your vac week.

You would have 13 days straight of vacation with 1 week. The real issue is that with the line bidding you can have 16+ days off in a row. Pick some OT at 150 or 200% and make the money you lost dropping two sequences.