Given the fact that this is a ER medication, you probably still need to be prescribed something for breakthrough pain.
Zohydro is an extended release formulation of pure hydrocodone (HC).
There are only two reasons you may still be having pain -- either your new dalily dose of HC is lower than you were getting with Vicodan, or the acetaminophen (APAP) was having more of an effect on your pain than either you or your doctor realized.
Check with your doctor on this, but doses of up to 4 grams of APAP per day are safe and not hepatotoxic unless you have liver impairment. (4 grams is equivalent to 8 x 500 mg tablets). You may be able to add some APAP back into your daily dose if it is giving you pain relief.