Outlook provides an easy and quick option to collaborate in a global team in terms of sharing forthcoming vacation/leaves with each other via 'Shared Calendars'.
You can create a new calendar and share it amongst the team members. Once shared, everyone gets the same instance of the calendar and any changes made in the calendar by any team member will instantly get reflected to all.
You can assign different privileges on the caldendar to the team members but the easiest is just allow everyone to edit it.
Here are the steps to use it -
You can even merge this calendar with the main calendar (by clicking arrow next to leaves calendar caption). In that case, you will see the leaves of your team mates as part of your main calendar itself.
Good luck. Feedback/Suggestions are welcome.