It means your too into looks, what's on the surface, designer labels...
Being vain implies pride and focus onto yourself, and your look... being in a relationship would be quite difficult for these individuals because relationships involve focus on the other.
These people usually suffer from a mild form of narcissistic personality disorder.
Their whole reality is self centered, every action of the outside world has something to do with them, they can't grasp the notion that there are billions of people on this planet with individual lives just like themselves.
A real turn off for relationship material, however probably not a bad thing for one night stands.