SYN useless:
She closed her eyes tightly in a vain attempt to hold back the tears. ? I knocked loudly in the vain hope that someone might answer.2(disapproving) too proud of your own appearance, abilities or achievements
SYN conceited:
She’s too vain to wear glasses.
—see also vanity(這是牛津最新版上的解釋,希望妳能看得懂!)