How long an opioid mediation stays in your system has little to do with getting off of it.  No opioid should be stopped suddenly or without being under a doctor's care because the withdrawal symptoms can be quite severe, and sometimes even dangerous.  It's important to know whether your daughter is addicted or dependent on Vicodin because there is a big difference between the two and a difference in the approach to getting off of it.  To learn more about how to tell the difference between addiction and dependence, read:  Opioids:  Addiction vs. Dependence 

If she is dependent and wants to quit taking Vicodin, her doctor should be able to help her gradually reduce her dosage to minimize withdrawal symptoms.  He can also prescribe medications to help with some of the symptoms, such as nausea.  There is also a new medication called Suboxone that can be quite effective in helping someone with chronic pain problems get off of strong opioids.  See Suboxone: An Exit Strategy

If she has a serious addiction problem, she might want to consider an in-patient treatment program.  For more information and help, call The National Alcohol and Substance Abuse Information Center hotline at 1-800-784-6776 in the U.S. and 1-215-784-1120 outside the U.S. or visit the NASAIC site at:  http://www.addictioncareoptions.com/