residence is slab on grade 2000 sq ft

attic has 4 inches of fiberglass insulation

no vapor barrier in attic between fiberglass and ceiling sheetrock attached
to rafters

will be adding 8" to 10" of blown in cellulose, should vapor barrier be
added between existing 4" fiberglass and drywall attached to the rafters,
before blowing the cellulose in?

or should no vapor barrier be used?