Once the vapor barrier or plastic is installed/stapled over the faced insulation while forgetting to destroy/slash insulation facing underneath, three likely possibilities exist for destroying/slashing the insulation facing beneath the plastic or vapor barrier.
1) Possible to pull-up the vapor barrier at the top and bottom, only destroying the vapor barrier at the points of stapling. After destroying/slashing the insulation facing, tape the vapor barrier using vapor barrier tape.
2) If only a few wall/ceiling cavities have faced insulation not destroyed/slashed, can slash through the tops and bottoms of the wall/ceiling cavity's vapor barrier AND insulation facing with a blade. Then, further cross slash the insulation facing in the event the vapor barrier tape adheres to the insulation facing closing the cut. Finish with taping the vapor barrier using vapor barrier tape.
TIP: Think ahead and make initial vertical slashes using the slashing method, and then the second slash destroying the insulation facing will then be good horizontal cut not adhere or closing after taping!
3) Completely forget about the slight oversight. If all else fails, blame your hired worker (for whom you paid good money) for forgetting to remind you the proper methods.
TODO: Not sure if required to entirely slash/destroy the entire faced insulation, or if just the tops and bottoms are necessary. Likely it is preferable to slash the entire facing. Likely depends on your climate, and your unique scenario building requirements.
TIP: Can destroy the entire facing by pulling a blade using a carpet shapped blade underneath a (clear) vapor barrier, with a string attached to the blade tool.
For my task, option number two was needed, as the ceiling plastic significantly overlapped areas, and I only had forgotten to destroy the insulation facing on several wall cavities.