I don't consider a T's vacation to be canceled sessions. My T gives me at least 2 weeks' notice of his vacations anyway. I've been seeing him for about 4 years now and I think he's never canceled a scheduled session with me (I know he had to with others but I expect it might have been just once or twice). He's asked me to change the time by 30 minutes a couple of times, the day before session, but that was it.