The Collaborative International Dictionary of English v.0.48:

Vacant \Va"cant\, a. [F., fr. L. vacans, -antis, p. pr. of
   vacare to be empty, to be free or unoccupied, to have
   leisure, also vocare; akin to vacuus empty, and probably to
   E. void. Cf. Evacuate, Void, a.]
   [1913 Webster]
   1. Deprived of contents; not filled; empty; as, a vacant
      room.
      [1913 Webster]

            Stuffs out his vacant garments with his form.
                                                  --Shak.
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            Being of those virtues vacant.        --Shak.
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            There is no fireside, howsoe'er defended,
            But has one vacant chair.             --Longfellow.
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   2. Unengaged with business or care; unemployed; unoccupied;
      disengaged; free; as, vacant hours.
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            Religion is the interest of all; but philosophy of
            those . . . at leisure, and vacant from the affairs
            of the world.                         --Dr. H. More.
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            There was not a minute of the day which he left
            vacant.                               --Bp. Fell.
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   3. Not filled or occupied by an incumbent, possessor, or
      officer; unoccupied; as, a vacant throne; a vacant house;
      a vacant apartment; a vacant parish.
      [1913 Webster +PJC]

            Special dignities which vacant lie
            For thy best use and wearing.         --Shak.
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   4. Empty of thought; thoughtless; not occupied with study or
      reflection; as, a vacant mind.
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            The duke had a pleasant and vacant face. --Sir H.
                                                  Wotton.
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            When on my couch I lie
            In vacant or in pensive mood.         --Wordsworth.
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   5. (Law) Abandoned; having no heir, possessor, claimant, or
      occupier; as, a vacant estate. --Bouvier.
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   Vacant succession (Law), one that is claimed by no person,
      or where all the heirs are unknown, or where all the known
      heirs to it have renounced it. --Burrill.
      [1913 Webster]

   Syn: Empty; void; devoid; free; unemployed; disengaged;
        unincumbered; uncrowded; idle.

   Usage: Vacant, Empty. A thing is empty when there is
          nothing in it; as, an empty room, or an empty noddle.
          Vacant adds the idea of having been previously filled,
          or intended to be filled or occupied; as, a vacant
          seat at table; a vacant office; vacant hours. When we
          speak of a vacant look or a vacant mind, we imply the
          absence of the intelligence naturally to be expected
          there.
          [1913 Webster]

WordNet (r) 3.0 (2006):

vacant
    adj 1: void of thought or knowledge; "a vacant mind"
    2: without an occupant or incumbent; "the throne is never
       vacant"

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "vacant":
   abandoned, absent-minded, available, bare, barren, bland, blank,
   blankminded, bleached, calm, characterless, clear, deadpan,
   deserted, devoid, dull, empty, empty-headed, empty-minded,
   empty-pated, empty-skulled, expressionless, extra, fatuous,
   featureless, fishy, forsaken, free, glassy, godforsaken, hollow,
   idle, impassive, inane, incogitant, inexpressive, insipid, jejune,
   nirvanic, null, null and void, oblivious, open, passive,
   poker-faced, quietistic, rattlebrained, rattleheaded, relaxed,
   scatterbrained, spare, stark, tenantless, thoughtfree, thoughtless,
   tranquil, unaware, uncomprehending, unengaged, unexpressive,
   unfilled, unideaed, uninhabited, unintellectual, unmanned,
   unoccupied, unpeopled, unpopulated, unreasoning, unrelieved,
   unstaffed, untaken, untenanted, untended, unthinking, unused,
   unutilized, vacuous, vapid, void, white, with nothing inside,
   without content, wooden