A very simple database would probably contain an employees table, an
AbsenceType table, and an EmployeeAbsences table.  It might need a few more,
depending on what type of information you want to get out of it.

Do you want to store the number of authorized vacation days?
Can employees roll their vacation days over from one year to the next?
What types of reports do you want to generate?

What type of system are you currently using to track employee information?  
Most professional HR systems come with at least a rudimentary absence
tracking capability.  If yours doesn't already have this, would you want to
link your Access database to that system?

HTH
Dale