Conscious of a vigil (5)
I believe the answer is:
awake
'conscious' is the definition.
(similar in meaning)
'a vigil' is the wordplay.
'vigil' becomes 'wake' (wake is a kind of vigil).
'a'+'wake'='AWAKE'
'of' is the link.
(Other definitions for awake that I've seen before include "Conscious, aware" , "Conscious and (usually) open-eyed" , "Alive and alert" , "Cease sleeping" , "Unable to sleep" .)