The Collaborative International Dictionary of English v.0.48:

Vehement \Ve"he*ment\, a. [L. vehemens, the first part of which
   is perhaps akin to vehere to carry, and the second mens mind:
   cf. F. v['e]h['e]ment. Cf. Vehicle, and Mental.]
   [1913 Webster]
   1. Acting with great force; furious; violent; impetuous;
      forcible; mighty; as, a vehement wind; a vehement torrent;
      a vehement fire or heat.
      [1913 Webster]

   2. Very ardent; very eager or urgent; very fervent;
      passionate; as, a vehement affection or passion. "Vehement
      instigation." --Shak. "Vehement desire." --Milton.
      [1913 Webster]

   Syn: Furious; violent; raging; impetuous; passionate; ardent;
        eager; hot; fervid; burning.
        [1913 Webster]

WordNet (r) 3.0 (2006):

vehement
    adj 1: marked by extreme intensity of emotions or convictions;
           inclined to react violently; fervid; "fierce loyalty";
           "in a tearing rage"; "vehement dislike"; "violent
           passions" [syn: fierce, tearing, vehement,
           violent, trigger-happy]
    2: characterized by great force or energy; "vehement deluges of
       rain"; "vehement clapping"; "a vehement defense"

Moby Thesaurus II by Grady Ward, 1.0:

131 Moby Thesaurus words for "vehement":
   abandoned, acid, acidulous, acrid, acrimonious, acute, afire,
   ardent, assiduous, astringent, biting, bitter, burning, caustic,
   committed, concentrated, cutting, dedicated, delirious, desperate,
   devoted, devout, diligent, double-edged, drastic, earnest, edged,
   emphatic, energetic, enthusiastic, escharotic, excessive, exciting,
   exorbitant, exquisite, extravagant, extreme, faithful, fervent,
   fervid, fierce, fiery, flaming, forceful, frantic, furious,
   glowing, great, hard, hardworking, harsh, hearty, heated, hot,
   hot-blooded, hotheaded, immoderate, impassioned, impetuous,
   in earnest, incisive, indefatigable, industrious, intemperate,
   intense, intent, intent on, keen, laborious, lively, loyal,
   mordacious, mordant, never idle, on fire, outrageous, passionate,
   penetrating, perfervid, piercing, poignant, potent, powerful,
   pronounced, provoking, rabid, red-hot, relentless, resolute,
   rigorous, rough, scathing, sedulous, serious, severe, sharp,
   simmering, sincere, sleepless, spirited, splitting, stabbing,
   stimulating, stinging, stirring, stormy, strenuous, strident,
   stringent, tart, tempestuous, terrible, tireless, trenchant,
   unconscionable, unflagging, unremitting, unsleeping, unsparing,
   unwearied, urgent, venomous, vicious, violent, virulent, vitriolic,
   volcanic, warm, white-hot, wild, zealous