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):

vigorous
    adj 1: characterized by forceful and energetic action or
           activity; "a vigorous hiker"; "gave her skirt a vigorous
           shake"; "a vigorous campaign"; "a vigorous foreign
           policy"; "vigorous opposition to the war"
    2: strong and active physically or mentally; "a vigorous old man
       who spent half of his day on horseback"- W.H.Hudson

Moby Thesaurus II by Grady Ward, 1.0:

199 Moby Thesaurus words for "vigorous":
   ablaze, acid, active, acute, afire, aggressive, animated, ardent,
   armipotent, athletic, authoritative, balmy, beefy, biting,
   blooming, blossoming, boiling over, booming, bouncing, breathless,
   brisk, burning, cartilaginous, chewy, clear, cogent, cohesive,
   cordial, coriaceous, corrosive, cutting, dashing, delirious,
   doughty, driving, drunk, durable, dynamic, effective, energetic,
   enterprising, enthusiastic, excited, exuberant, fair, fat, febrile,
   fervent, fervid, fevered, feverish, fibrous, fiery, fit, flaming,
   flourishing, flowering, flush, flushed, forceful, forcible, forcy,
   fruiting, full of beans, full of pep, full-blooded, full-strength,
   glowing, go-go, going strong, gristly, gutsy, gutty, halcyon, hale,
   hale and hearty, hard, hard as nails, hard-hitting, hardy, healthy,
   hearty, heated, hefty, high-potency, high-powered, high-pressure,
   high-tension, hot, husky, impassioned, imperative, impetuous,
   impressive, in force, in full swing, in good case, in power,
   incisive, intense, intoxicated, iron-hard, irresistible, keen,
   kinetic, lasting, leatherlike, leathery, lively, living, lusty,
   masterful, mettlesome, mighty, mighty in battle, mordant, muscular,
   nervous, nervy, obstinate, on fire, operative, palmy, passionate,
   penetrating, peppy, piercing, piping, poignant, potent, powerful,
   prepotent, prospering, proud, puissant, punchy, red-blooded,
   red-hot, resilient, resistant, robust, robustious, robustuous,
   ropy, rosy, rough-and-ready, rude, rugged, ruling, sensational,
   sinewed, sinewy, slashing, sleek, smacking, snappy, spanking,
   spirited, sprightly, spry, stalwart, steaming, steamy, steely,
   stiff, stout, strapping, strenuous, striking, stringy, strong,
   strong as brandy, strong as strong, strong-willed, stubborn,
   sturdy, take-charge, take-over, telling, tenacious, thriving,
   tough, tough as leather, trenchant, unrestrained, untiring, valid,
   vibrant, vigil, viscid, vital, vivacious, vivid, warm, wiry,
   zealous, zestful, zesty, zippy