I would like to know to what extent vanity is a sin, both in men and women.
Vanity in the strict sense consists in exceeding the just measure in dress and appearance; exceeding what is suitable, given the conditions of person, place, and time, or giving too much time and care to the pose of the body. Since vainglory is the end of vanity, it is also sometimes called vanity.
As far as morality is concerned, vanity is in itself a venial sin. However, it can become a mortal sin when:
a) there is a serious scandal that severely harms the family or makes it impossible for the one acting vain to pay their debts (the ones someone incurs in order to maintain their vanity);
b) when an end in serious opposition to the moral law is intended.
For further information on the subject see: Dictionary of Moral Theology, edited by Cardinal Francesco Roberti (The Newman Press, 1962; ‘vanity’).
Fr. Miguel A. Fuentes, IVE