You can't. If you are allergic to the active ingredients in acetominophen, you should ask your doctor for a different medication instead of playing chemist at home.
Vicodin already contains acetaminophen. It would not be wise to take additional acetaminophen if you are taking Vicodin already, as excess acetaminophen can be harmful to your liver.
Look at the label on your prescription. It probably says something like 5/500. The 5 represents 5 mg of vicodin and the 500 represents 500 mg of acetaminophen. The maximum daily dose of acetamiophen...