(1):

(superl.) Destitute of forge or efficacy; effecting no purpose; fruitless; ineffectual; as, vain toil; a vain attempt.

(2):

(superl.) Showy; ostentatious.

(3):

(n.) Vanity; emptiness; - now used only in the phrase in vain.

(4):

(superl.) Having no real substance, value, or importance; empty; void; worthless; unsatisfying.

(5):

(superl.) Proud of petty things, or of trifling attainments; having a high opinion of one's own accomplishments with slight reason; conceited; puffed up; inflated.