The Collaborative International Dictionary of English v.0.48:

Vigorous \Vig"or*ous\, a. [Cf. OF. vigoros, F. vigoureux, LL.
   vigorosus.]
   1. Possessing vigor; full of physical or mental strength or
      active force; strong; lusty; robust; as, a vigorous youth;
      a vigorous plant.
      [1913 Webster]

            Famed for his valor, young,
            At sea successful, vigorous and strong. --Waller.
      [1913 Webster]

   2. Exhibiting strength, either of body or mind; powerful;
      strong; forcible; energetic; as, vigorous exertions; a
      vigorous prosecution of a war.
      [1913 Webster]

            The beginnings of confederacies have been always
            vigorous and successful.              --Davenant.
      [1913 Webster] -- Vig"or*ous*ly, adv. --
      Vig"or*ous*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

vigorously
    adv 1: with vigor; in a vigorous manner; "he defended his ideas
           vigorously" [syn: vigorously, smartly]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "vigorously":
   actively, alertly, boldly, briskly, dynamically, eagerly,
   energetically, firmly, forcefully, forcibly, hammer and tongs,
   hardily, hardly, heartily, like crazy, like mad, lustily,
   might and main, mightily, powerfully, purposefully, resolutely,
   robustly, spiritedly, stalwartly, strenuously, strongly,
   unfalteringly, vivaciously, with a vengeance, zealously