Hitchcock's film is hard to fault - I found the comic relief sequences tended to jar with the rest of the film but overall The Lady Vanishes is very effective psychodrama. Flightplan is seriously flawed - supposedly post 9/11 however this is only true superficially - a knee-jerk reaction for many of the passengers is to blame a group of "suspicious" Arabs for the mysterious happenings. For some reason these curious characters are called upon to apologise to Jodie Foster later in the film - I'm trying not to think too hard about what's being said symbolically here. Once the mystery is uncovered things actually degenerate into the kind of high-altitude shenanigans that could only really happen before 9/11 as these days the plane would probably have been shot down about half an hour before the end of the film. And, this is a newly designed, high security plane. You can't get in or out of the cockpit - but it seems you can easily get from the toilet cubicles into the cargo space (admittedly if you, the heroine, happen to have designed the plane).
For all this, the tension is maintained by the quality of the acting and camerawork, making excellent use of a large but claustrophobic set.