See for example the ceiling question:
http://www.handymanusa.com/articles/moisture.html

Anything you install should be on the warm side which means you should life the insulation up over your fixture rather then cut a hole in the face (vapor barrier) of the insulation.
However you need to check the lighting instructions, some may get too warm to directly touch the insulation or the face of the insulation so you will need to frame around it by however much space they require. So if the light required 4" you would build a frame 4" around and 4" above then cover with plywood or chipboard and then the insulation over that.