Should I install a plastic vapor barrier below the insulation between my ceiling and attic floor?
Air sealing is more important than vapor sealing.  All assemblies (wall and ceiling) need to be able to dry out.  If you prevent air passage AND provide sufficient insulation to prevent condensation, you don't need or want an impervious layer.  

Cables,plumbing vents, ducts and lighting fixtures are notorious sources for air leaks.  These can be stopped with many materials, including caulk, expanding foam and fire stop (special foam or mineral wool), as appropriate.