Until quite recently, most people equated narcissism with vanity. The understanding was that a narcissist was someone who was overly concerned with their outward appearance. For many people, that is still their perception.
However, there is a recognized psychological condition called Narcissistic Personality Disorder, and those suffering from it are called narcissists.
Narcissists, so defined, have a great many more traits, symptoms, and related behaviours than mere vanity.
Narcissistic Personality Disorder is used to describe someone who exhibits an unhealthy fixation with themselves to the detriment of others. They lack the ability to have a normal sense of empathy for other people.
The person who displays these characteristics will not be able to contribute to a healthy relationship, and as a result, their behaviour will be toxic to their partner.