The Collaborative International Dictionary of English v.0.48:
Vacuousness \Vac"u*ous*ness\, n. The quality or state of being vacuous; emptiness; vacuity. --W. Montagu. [1913 Webster]WordNet (r) 3.0 (2006):
vacuousness n 1: indicative of or marked by mental vacuity and an absence of ideas; "the vacuousness of her face belied her feelings"