Is it safe to use apple cider vinegar / baking soda to remove mold on (soft) plastic?
Depending upon your mix, the PH may wind up at 7.0, exactly neutral. Vinegar (acetic acid) lowers the PH, baking soda (sodium bicarbonate) raises the PH.

Either is GENERALLY harmless to MOST soft plastics, whether alone or in combination. Do you have a specific plastic in mind? If it happens to be polyethylene ("polythene", to those on the opposite side of the Big Blue Wobbly Thing from me), that's nearly perfectly impervious to both vinegar and baking soda.