Use vigilant to describe someone who keeps awake and alert in order to avoid danger or problems. When taking the subway, be vigilant about your wallet — always know where it is, or someone might steal it from you.
Vigilant descends from the Latin vigilare, "to be awake, watch," from vigil, "awake, alert." If you stay awake vigilantly all night, you're keeping watch or keeping a vigil. A vigilante (pronounced vij-uh-LAN-tee) is a person who acts outside the legal system to punish crime.