Same word as vigilant:

acute, agog, alert, anxious, attentive, aware, cautious, circumspect, guarded, keen, looking for, looking to, observant, on alert, on guard, on the ball, on the job, on the lookout, on the qui vive, on toes, open-eyed, sharp, sleepless, unsleeping, waiting on, wakeful, wary, wide-awake, with eyes peeled, with weather eye open