How do I remove defrost vent on dash?

A 4-in crescent will not fit in there it's tight I think maybe starting to snap off the top might be an idea

Sure it will, I did it that way with my own car.:D
point the wrench straight at the nut and turn it one flat at a time instead of trying to get it on the nut like normal and turn it all the way around.

Also, on a part you will never use again you can snap off the stud with the nut still on it.
Sometimes they snap right where the nut cut into the plastic stud.