I've found a solution :-) .
Before to execute the "Add" method I have to delete the Exception at the same period of my vacation like below
'''
Set cal = ActiveProject.Resources(resourceName).Calendar
For j = cal.Exceptions.Count To 1 Step -1
If cal.Exceptions(j).Start >= startDate And cal.Exceptions(j).Finish <= endDate Then
cal.Exceptions(j).Delete
End If
Next j