The Collaborative International Dictionary of English v.0.48:

Unvirtuous \Unvirtuous\
   See virtuous.

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "unvirtuous":
   Adamic, backsliding, besmirched, carnal, defiled, erring, fallen,
   fleshly, frail, impure, indecent, infirm, lapsed, maculate,
   of easy virtue, peccable, postlapsarian, prodigal, recidivist,
   recidivistic, smirched, soiled, sullied, tainted, unangelic,
   unchaste, unclean, ungodly, ungood, unrighteous, unsaintly,
   unvirginal, virtueless, wanton, wayward, weak