We cover every light box and hole before blowing cellulose in an attic. I blew cellulose in houses for years and have it in my own house. I wish I didn't. The dust will find every way it can into your living space and regardless of what they say some people are allergic to the dust. Do a search and you will see. I would stick with the fiberglass, it stays where you put it.
As for vapor barriers, there is code, inspectors usually know best? What is best? Ask a hundred people and get a hundred different answers. IMO, If you put too much vapor barrier in a house, walls ceilings and everywhere, you will have moisture problems and will need an air exchanger to solve the problem. I like a vapor barrier in the walls and none in the attic with lots of attic vents. We heat more than cool here, environment does determine vapor barrier specs.