Vigils in the dark or vigils in the light....
The group i used to do investigations with did vigils with lights on, unless outside of course.
One of the first indoor vigils i was part of used full lighting downstairs and muted lighting in the upstairs area, just the wall lights not the ceiling ones so we could see what we were doing and not fall over anything whilst moving around. All in all a good vigil, we got sounds from upstairs where myself and my team were without our shoes on so as not to make any noise for the people downstairs to hear. We were asked what were we were dragging across the floor.
I went on an investigation with another group who did them in the dark, or more accuratly what they said would be normal lighting conditions i.e. If a light is normally on it stays on as in lets say a corridor light in a hotel. But in individual rooms at 1am the occupant would probably be asleep so the light would be off (unless they are scared of the dark). The mind doesnt half play tricks when your awake at 2-3am and can't see your hand in front of your face, or are you more receptive
Both approches have their own merits, on one hand if you have the lights on you are less likely to make noise falling over something you cant see so less chance of something inadverantly going bump in the night, and if you have the lights off or "normal lighting" you would get a better impression of what if anything has been seen by previous sightees.
The really big question is, why do ghosts only come out at night ? Do we regress to a primal stage where the dark contains all sorts of horrible beasties that will drag us out of our caves and eat us alive ?