vain

If you are vain you think about yourself a lot, specially how you look.

Example: She was so vain she spent hours looking at herself in the mirror.

If something is vain, it means it was not successful.

Example: She made a vain attempt to feed the baby carrots, but he kept spitting them out into her face.

If something is in vain, it means it happened for no reason or had no purpose.

Example: The Prime Minister said that the soldiers killed in the battle did not die in vain. They helped save many innocent people.