Vanity is a narcissistic trait and is born out of a low self esteem as a protective mechanism to prevent further pain and gain approval. Too little narcissism leaves an individual with no self confidence and little self esteem and leads to fearfulness and feelings of inferiority. On the other hand too much can result in an inflated ego and a core belief that they are better than everyone else. The ideal is to have a healthy balance where people feel good about themselves without compromising their social relations. As long as this behavior is not coupled with other narcissistic traits and does not affect your relationship then it is just an overdeveloped trait and still within a healthy/normal range. The defensive attitude they take regarding their vanity is also a protective behavior designed to keep them feeling good about themselves and is in keeping with a narcissistic type of behavior. They do not like criticism, since this is perceived as a threat to their ego and will shoot down the opposition in a flash. Telling them they are overly vain is a criticism which is why you are met with a rather abrupt black and white answer. Hope this helps you understand a bit more.