To be vain: to have an excessively high opinion of oneself & thinking to be superior to other; synonymy of narcissistic, egocentric, etc. Usually a negative trait to have as a person since it drives away everyone away due to the naturally poor treatment others get by a vain person. It may cause the person harm in the long run. In short: an individual who thinks too unnecessarily highly of themselves for their own good.
The word 'vain' has two very different definitions. 1. To think highly/excessively of oneself, to look down on others (in terms of appearance, abilities, worth, etc.) Example: Two much praise can make one vain 2. A useless result of something/no result Example: She made a vain attempt to clean the car Both definitions are adjectives.