The Collaborative International Dictionary of English v.0.48:

Vain \Vain\ (v[=a]n), a. [Compar. Vainer; superl. Vainest.]
   [F. vain, L. vanus empty, void, vain. Cf. Vanish, Vanity,
   Vaunt to boast.]
   1. Having no real substance, value, or importance; empty;
      void; worthless; unsatisfying. "Thy vain excuse." --Shak.
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            Every man walketh in a vain show.     --Ps. xxxix.
                                                  6.
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            Let no man deceive you with vain words. --Eph. v. 6.
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            Vain pomp, and glory of this world, I hate ye!
                                                  --Shak.
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            Vain wisdom all, and false philosophy. --Milton.
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   2. Destitute of force or efficacy; effecting no purpose;
      fruitless; ineffectual; as, vain toil; a vain attempt.
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            Bring no more vain oblations.         --Isa. i. 13.
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            Vain is the force of man
            To crush the pillars which the pile sustain.
                                                  --Dryden.
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   3. Proud of petty things, or of trifling attainments; having
      a high opinion of one's own accomplishments with slight
      reason; conceited; puffed up; inflated.
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            But wilt thou know, O vain man, that faith apart
            from works is barren?                 --James ii. 20
                                                  (Rev. Ver.).
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            The minstrels played on every side,
            Vain of their art.                    --Dryden.
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   4. Showy; ostentatious.
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            Load some vain church with old theatric state.
                                                  --Pope.
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   Syn: Empty; worthless; fruitless; ineffectual; idle; unreal;
        shadowy; showy; ostentatious; light; inconstant;
        deceitful; delusive; unimportant; trifling.
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The Collaborative International Dictionary of English v.0.48:

Vain \Vain\, n.
   Vanity; emptiness; -- now used only in the phrase

   in vain.
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   For vain. See In vain. [Obs.] --Shak.

   In vain, to no purpose; without effect; ineffectually. " In
      vain doth valor bleed." --Milton. " In vain they do
      worship me." --Matt. xv. 9.

   To take the name of God in vain, to use the name of God
      with levity or profaneness.
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WordNet (r) 3.0 (2006):

vain
    adj 1: characteristic of false pride; having an exaggerated
           sense of self-importance; "a conceited fool"; "an
           attitude of self-conceited arrogance"; "an egotistical
           disregard of others"; "so swollen by victory that he was
           unfit for normal duty"; "growing ever more swollen-headed
           and arbitrary"; "vain about her clothes" [syn:
           conceited, egotistic, egotistical, self-
           conceited, swollen, swollen-headed, vain]
    2: unproductive of success; "a fruitless search"; "futile years
       after her artistic peak"; "a sleeveless errand"; "a vain
       attempt" [syn: bootless, fruitless, futile,
       sleeveless, vain]

Moby Thesaurus II by Grady Ward, 1.0:

147 Moby Thesaurus words for "vain":
   Gascon, abortive, absurd, airy, arrogant, asinine, barren,
   baseless, boastful, boasting, bootless, bootlessly, braggart,
   bragging, catchpenny, cocky, complacent, conceited, consequential,
   counterproductive, coxcombical, dandyish, delusive, delusory,
   doomed, effete, egocentric, egoistic, egotistical, empty,
   etiolated, fanfaron, fanfaronading, fatuitous, fatuous, feckless,
   flimsy, foolish, foppish, foredoomed, fribble, fribbling,
   frivolous, frothy, fruitless, fruitlessly, futile, futilely,
   gasconading, groundless, haughty, hollow, idle, ill-founded,
   improperly, in vain, inadequate, inane, ineffective, ineffectual,
   inefficacious, inoperative, invalid, light, misleading, narcissan,
   narcissine, narcissistic, narcistic, nugacious, nugatory,
   of no force, otiose, overproud, overweening, paltry, petty,
   pointless, profitless, proud, puny, self-admiring,
   self-advertising, self-applauding, self-approving, self-assuming,
   self-complacent, self-conceited, self-congratulating,
   self-congratulatory, self-content, self-contented, self-delighting,
   self-endeared, self-esteeming, self-flattering, self-glorious,
   self-gratulating, self-gratulatory, self-important, self-lauding,
   self-loving, self-respecting, self-satisfied, self-sufficient,
   self-vaunting, shallow, silly, slender, slight, smug, sterile,
   stuck-up, superficial, thrasonic, thrasonical, trifling, trite,
   trivial, unavailable, unavailing, unbased, unfounded, ungrounded,
   unproductive, unprofitable, unsuccessful, unsuccessfully,
   unsupportable, unsupported, unsustainable, unsustained, untenable,
   unwarranted, useless, vacuous, vainglorious, vainly, valueless,
   vapid, vaporing, vaunting, void, windy, without basis,
   without foundation, worthless