The Collaborative International Dictionary of English v.0.48:

Vacuous \Vac"u*ous\ (v[a^]k"[-u]*[u^]s), a. [L. vacuus. See
   Vacant.]
   Empty; unfilled; void; vacant.
   [1913 Webster]

         Boundless the deep, because I am who fill
         Infinitude; nor vacuous the space.       --Milton.
   [1913 Webster]

         That the few may lead selfish and vacuous days. --J.
                                                  Morley.
   [1913 Webster]

WordNet (r) 3.0 (2006):

vacuous
    adj 1: devoid of intelligence [syn: asinine, fatuous,
           inane, mindless, vacuous]
    2: devoid of significance or point; "empty promises"; "a hollow
       victory"; "vacuous comments" [syn: empty, hollow,
       vacuous]
    3: devoid of matter; "a vacuous space"
    4: void of expression; "a blank stare" [syn: blank, vacuous]

Moby Thesaurus II by Grady Ward, 1.0:

111 Moby Thesaurus words for "vacuous":
   airy, asinine, awkward, bare, barren, bland, blank, blankminded,
   bleached, callow, calm, catchpenny, characterless, clear, devoid,
   dumb, empty, empty-headed, empty-minded, empty-pated,
   empty-skulled, existless, fatuous, featureless, flimsy, foolish,
   fribble, fribbling, frivolous, frothy, futile, gauche, green,
   groping, hollow, idle, ignorant, inane, incogitant, inexperienced,
   innocent, insipid, jejune, know-nothing, lacking, light, minus,
   missing, naive, negative, nescient, nirvanic, nonexistent,
   nugacious, nugatory, null, null and void, oblivious, otiose,
   passive, quietistic, rattlebrained, rattleheaded, raw, relaxed,
   scatterbrained, shallow, silly, simple, slender, slight,
   strange to, superficial, tentative, thoughtfree, thoughtless,
   tranquil, trifling, trite, trivial, unacquainted, unapprized,
   uncomprehending, unconversant, unenlightened, unexisting,
   unfamiliar, unideaed, unilluminated, uninformed, uninitiated,
   unintellectual, unintelligent, unknowing, unoccupied, unposted,
   unreasoning, unrelieved, unripe, unsure, unthinking, unversed,
   vacant, vain, vapid, void, white, windy, with nothing inside,
   without being, without content