The Collaborative International Dictionary of English v.0.48:

Vanadous \Van"a*dous\, a. (Chem.)
   Of or pertaining to vanadium; obtained from vanadium; -- said
   of an acid containing one equivalent of vanadium and two of
   oxygen; specifically, designating those compounds in which
   vanadium has a lower valence as contrasted with the vanadic
   compounds; as, vanadous acid [Sometimes written also
   vanadious.].
   [1913 Webster +PJC]

The Collaborative International Dictionary of English v.0.48:

Vanadious \Va*na"di*ous\, a. (Chem.)
   Pertaining to, or containing, vanadium; specifically,
   designating those compounds in which vanadium has a lower
   valence as contrasted with the vanadic compounds; as,
   vanadious acid. Usually written vanadous. [Sometimes
   written also vanadous.]
   [1913 Webster]