Vanity (in the sense of excessive concern for our own appearance) makes us ugly.
At conception (in the sense of coming up with ideas and making decisions), vanity stops us from taking life-giving risks (which expose us to the possibility of failure); it is sterilising.
At execution (in the sense of carrying things out), vanity drives us to a kind of perfectionism where we seek to control both our own performance and how people receive it, stifling the joy of both by killing spontaneity and responsiveness; it is suffocating.
On reflection (as we look back on our performance), vanity distorts our vision by exaggerating both our strengths and our shortcomings; it is a hall of mirrors.