how long does it take for vicodin to get out of system if taken once?

If you are asking how long it takes for a particular drug to get out of your system for any reason other than passing a drug test, call your doctor or pharmacist for that information.  If you are concerned about passing a drug test, following is the information you need. 

If you are taking a drug that has been prescribed for you, all you have to do is take your prescription with you to the drug test and there should be no problem.  If you are taking drugs other than those prescribed for you, the best thing we can do for you is encourage you to get any help you need to stop taking them as soon as possible.  For 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/

For more information about employment-related drug screenings, read: "Passing" Those Drug Screenings
http://www.healthcentral.com/chronic-pain/c/5973/40834/screenings