“It is worth stressing that in the grant of vacation and sick leave privileges to an employee, the employer is given leeway to impose conditions on the entitlement to the same as the grant of vacation and sick leave is not a standard of law, but a prerogative of management. It is a mere concession or act of grace of the employer and not a matter of right on the part of the employee. Thus, it is well within the power and authority of an employer to deny an employee’s application for leave and the same cannot be perceived as discriminatory or harassment.”