The Collaborative International Dictionary of English v.0.48:

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

   in vain.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

in vain
    adv 1: to no avail; "he looked for her in vain"; "the city
           fathers tried vainly to find a solution" [syn: vainly,
           in vain]