The problem with vanity is that it leads us down the wrong path. The mirror helps us get to know ourselves but only on the outside, and what we look like is only the representative intermediary of the character on the inside - the emotive, feeling, thinking, alive being. The part that animates the outside is the inside.
When we reflect, it is not ever to look in the mirror and say, "that is who I am", but to think of the past and see who we are based on what we've done, good and bad.
We have no choice as to what we look like or the family we grow up in or the country we are from yet we value these things as if they were qualities. The little choice we have is entirely internal. Sure, sure, we can decorate our outside with trinkets and cloth and animal hides and design our hair the best we can, we can exercise and attempt to stay healthy. True choice is how we react to each situation, how much we know ourselves, what we will dedicate ourselves to learning. And of course, because we wear and tarnish, everything physical will erode, shrink and dissapear, everything invested towards the outside is in vain, hence the adjective - VAIN. The internal is what will last, what we will remain until the very last second of life, and after that, who knows.
There are two things - the outside of the individual; being other people, the weather, all sound and light vibration, and what we look like (acting as a representative intermediary of who we are and demonstrating that we co-exist with others.) Then there is the inside which are thoughts, feelings, emotions, fears, desires, aspirations, disappointments (essentially - memories, the present manifestation, and hopes), health, etc,.
If the outside representation is nurtured then the things listed under 'inside' are consequently diminished; such as having no self awareness, confusion, feeling lost and thus not understanding others simply because we misinterpret ourselves.
Of course, knowing the self is the hardest thing in the world and takes a lifetime (some say multiple lifetimes). Simple objective self-observation is the most effective means of developing the inside just following meditation - which trains the objective, observatory mind.