Microsoft Outlook and Microsoft Project are different things. Outlook
is a personal information manager, whereas Project is a project
management computational tool. Accordingly, what you want to do is not
really directly possible simply because it's not a feature provided by
Microsoft.
You can account for people's unavailability to work on a project in
Project by modifying the individual's resource calendar. Show the dates
of unavailability due to vacations there. This can be done manually in
Project. You can read about Resource calendars in Project's Help.
I guess it would be possible to write a program that would import, or
facilitate import, of Outlook calendar data into Project. That would
take a lot of programming skills and the right tools.
Hope this is useful to you. Let us know.
rms