Under "My Settings" above use "My Vaction Settings". On the left side of the calender page are the instructions:
Existing booked vacations are shown as highlighted days with a heavy border.
Vacations that are yet to start can be cancelled simply by clicking them.
All vacations are booked using GMT which may not match your time zone precisely.
There is a cut-off point after which you can't cancel, but I'm not sure what it is, maybe midnight Zulu time on the day of your first vacation day?