The Collaborative International Dictionary of English v.0.48:

Vastness \Vast"ness\, n.
   The quality or state of being vast.
   [1913 Webster]

WordNet (r) 3.0 (2006):

vastness
    n 1: unusual largeness in size or extent or number [syn:
         enormousness, grandness, greatness, immenseness,
         immensity, sizeableness, vastness, wideness]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "vastness":
   ampleness, amplitude, boundlessness, bulk, enormity, enormousness,
   expanse, formidableness, fullness, gigantism, grandeur, grandness,
   great scope, greatness, hugeness, immensity, infinity, intensity,
   largeness, magnitude, might, mightiness, monstrosity,
   monstrousness, muchness, plenitude, power, prodigiousness,
   strength, stupendousness, tremendousness