The Collaborative International Dictionary of English v.0.48:

Vigilant \Vig"i*lant\, a. [L. vigilans, -antis, p. pr. of
   vigilare to watch, fr. vigil awake: cf. F. vigilant. See
   Vigil.]
   Attentive to discover and avoid danger, or to provide for
   safety; wakeful; watchful; circumspect; wary. "Be sober, be
   vigilant." --1 Pet. v. 8.
   [1913 Webster]

         Sirs, take your places, and be vigilant. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

vigilant
    adj 1: carefully observant or attentive; on the lookout for
           possible danger; "a policy of open-eyed awareness"; "the
           vigilant eye of the town watch"; "there was a watchful
           dignity in the room"; "a watchful parent with a toddler
           in tow" [syn: argus-eyed, open-eyed, vigilant,
           wakeful]

Moby Thesaurus II by Grady Ward, 1.0:

89 Moby Thesaurus words for "vigilant":
   Argus-eyed, acute, agog, alert, all ears, all eyes, all ready,
   all set, armed, armed and ready, attentive, aux aguets, avid,
   booted and spurred, briefed, careful, cautious, chary, circumspect,
   coached, cocked, covering, custodial, defensive, eager, eagle-eyed,
   equipped, familiarized, fostering, good and ready, groomed,
   guarded, guardian, guarding, hawk-eyed, immunizing, in arms,
   in battle array, in readiness, in the saddle, informed, keen,
   keen-eyed, lidless, loaded, loaded for bear, mature, mobilized,
   observant, on guard, on the alert, on the lookout, on the mark,
   on the watch, open-eyed, parental, planned, prearranged, prepared,
   prepared and ready, prepped, preventive, primed, prophylactic,
   protecting, protective, provided, prudent, psyched up, ready,
   ready for anything, ripe, safeguarding, screening, set, sharp,
   sharp-eyed, sheltering, shielding, sleepless, tutelary, unsleeping,
   up in arms, wakeful, wary, watchful, well-prepared, wide-awake,
   with open eyes