The Collaborative International Dictionary of English v.0.48:

Vigor \Vig"or\, n. [OE. vigour, vigor, OF. vigor, vigur, vigour,
   F. vigueur, fr. L. vigor, fr. vigere to be lively or strong.
   See Vegetable, Vigil.]
   1. Active strength or force of body or mind; capacity for
      exertion, physically, intellectually, or morally; force;
      energy.
      [1913 Webster]

            The vigor of this arm was never vain. --Dryden.
      [1913 Webster]

   2. Strength or force in animal or vegetable nature or action;
      as, a plant grows with vigor.
      [1913 Webster]

   3. Strength; efficacy; potency.
      [1913 Webster]

            But in the fruithful earth . . .
            His beams, unactive else, their vigor find.
                                                  --Milton.
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   Note: Vigor and its derivatives commonly imply active
         strength, or the power of action and exertion, in
         distinction from passive strength, or strength to
         endure.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Vigor \Vig"or\, v. t.
   To invigorate. [Obs.] --Feltham.
   [1913 Webster]

WordNet (r) 3.0 (2006):

vigor
    n 1: forceful exertion; "he plays tennis with great energy";
         "he's full of zip" [syn: energy, vigor, vigour,
         zip]
    2: active strength of body or mind [syn: vigor, vigour,
       dynamism, heartiness]
    3: an imaginative lively style (especially style of writing);
       "his writing conveys great energy"; "a remarkable muscularity
       of style" [syn: energy, muscularity, vigor, vigour,
       vim]

Moby Thesaurus II by Grady Ward, 1.0:

165 Moby Thesaurus words for "vigor":
   amperage, animal spirits, animation, armipotence, authority, bang,
   beef, bite, bitingness, black power, bounce, brio, briskness,
   brute force, capacity, capersomeness, charge, charisma, clout,
   cogence, cogency, coltishness, compulsion, cuttingness,
   decisiveness, demonic energy, dint, drive, duress, dynamism,
   eagerness, effect, effectiveness, effectuality, elan, endurance,
   energy, enthusiasm, ergal, esprit, exuberance, flower power, force,
   force majeure, forcefulness, fortitude, friskiness, frolicsomeness,
   full blast, full force, gaiety, gamesomeness, gayness,
   get-up-and-go, getup, go, gusto, guts, gutsiness, haleness,
   hardiness, healthiness, heartiness, impressiveness, incisiveness,
   influence, intensity, intestinal fortitude, kinetic energy, life,
   liveliness, longevity, lustihood, lustiness, main force,
   main strength, mana, manliness, mettle, mettlesomeness, might,
   might and main, mightiness, mordancy, moxie, muscle, muscle power,
   muscularity, nervosity, nervousness, obstinacy, oomph, pep,
   piss and vinegar, pith, pizzazz, playfulness, poignancy, poop,
   potence, potency, potential energy, potentiality, power,
   power pack, power structure, power struggle, powerfulness,
   prepotency, productiveness, productivity, puissance, pull, punch,
   push, resilience, robustness, rollicksomeness, rompishness,
   ruggedness, sinew, sinewiness, skittishness, snap, soundness,
   spirit, spiritedness, spirits, sportiveness, sprightliness, spunk,
   stalwartness, stamina, starch, staying power, steam,
   sticking power, stoutness, strength, strength of will,
   strenuousness, strong arm, strong language, sturdiness,
   superiority, superpower, toughness, trenchancy, validity,
   vehemence, verve, vigorousness, vim, virility, virtue, virulence,
   vitality, vivacity, wattage, weight, zealousness, zest,
   zestfulness, zing, zip