Database Design Review
First identify the entities in your design (in hierarchical order, i.e. in dependency):


Department
User
VacationRequest


Then identify the relationships:


Each Department have one or more User 
Each User can have 0 or more VacationRequest


Then the business rules:


Each Department must have one responsible and one deputy.
If the responsible of the department have a confirmed vacation then in the same period then the deputy can't have a confirmed vacation (and viceversa) 


How the rules could be implemented:


Create the tables Department, User, VacationRequest.
In the user table assign the user as responsible of his own department or deputy.
Each time you enter a new VacationRequest asign it with the Confirmed field on false.
Each time you confirm a Vacation via a stored procedure you must check if the vacations not colide with the deputy or viceversa (also you must check if there is a deputy effectively)


I hope it would help you.