If it was me, I would inform that I was planning on leaving. That way they can plan to have someone on board when you leave, and everybody is happy because they had time to plan everything out. They should think highly of you for doing it that way. If they want to fire you and not give you vacation at that point, well they are just a bad operation and you don't want to be there anyway. I would be willing to guess that it goes the way of the first part of my post, not the second.