I use to work as a pharmaceutical analytical chemist, but not this type of drug screening work, but maybe I can give you some insight that could help you.
First of all the drug vicoprofen has a greater abuse potential than propoxyphene so in terms of motive, if you have vicoprofen why mess with the other stuff?
One possibility is that you were given the wrong drug. Currently there is 1 one brand name drug (vicoprofen) and three generics that could have been substituted.
Look under therapeutic equivelants at the fda web site for a list of the other manufactures. (Hopefully no imported generic fakes.)
So, check this first.
There are questions to ask. When you took the test had you been taking the vicoprofen within the past day. If so, did the two drugs which are in it show up? If not that should indicate that something is wrong with their testing process.
Another thing is you need to find out what kind of test was run.
The cheapest type of test are chemical tests that involve color change or something like that. They are not specific, are are really only good to tell you that something might be there, not that something is absolutelty positively there.
The other types of tests involve chromatography, there are various types, thin layer, liquid, gas, ect. What chromatography does is separate substances of a mixture into their individual components. The cheapest, and most prone to error way to use a chromatography test is just to see how long or how far a substance travels to tell you what it is. Minor changes in solvent can make things come out at the same time as a contolled substance.
That is why, it is federal law that government workers have to have beyound the chromatographic separation of the substances follow up with Mass spectra (which give the molecular weight of the substance) and IR specta, which gives the 'fingerprint' for the substance.
When the drugs you take are ingested, the human body transforms them into other compounds. This is another reason why the tests given should follow up with the MS and IR tests.
When I was living in Lafayette Indiana a few years ago a woman who took an Rx drug with hydrocodone in it (like vicoprofen has) had a false positive test for heroin due to the cheap chemical test the court used, and took her kids away.
I read in a journal from a drug testing firm that ibuprofen came out at the same time as THC but the employer knew the character of the woman so challanged the company to investigate it. The company modified the procedure by adding an extra chromatographic separation, but they did not use IR or MS.
Their idea to avoid false positive was to make a chromatographic test process with a long, long list of drugs, controlled and not controlled to avoid false positives. But again, it ignore the metabolic breakdown products that people make when they process drugs in their system, which allows false positives as a real possibility from metabolites they probably could not even buy on the market to include in their testing procedures.
I am no lawyer, but it seems from a legal standpoint, if companies who sell these services or subscribe to these services don't use the follow up MS and IR for proof, whereas for government employees (like police and firemen) get these protections, it is unequal protection under the law, sounds like a constitutional violation of rights but again I am no lawyer.
It could be just bad quality control, and you got your sample mixed in with somebody else, again did they find ibuprofen or hydrocodone? If not, something is screwed if you had it in your system.
PS: look at the Fda website for a list of drugs with propoxyphene,under drugs@fda
If your employer would otherwise have to pay you unemployement benefits but is wesseling out of it because they fired you with this drug test, then should of course appeal to the unemployment office. If the unemployment office refuses to give you benefits, my guess is then you could sue in court for violation of your due process rights of being able to face your accuser and the supporting evidence the accuser has, so they can be cross examined by your lawyer (or you if you represent your self) and the other reason of unequal protection under the law (see if the drug testing firm gives the MS/IR testing for some clients, but not you. Did the state unemployment office require the drug testing firm appear at your hearing? If not the state is also at fault.Again I am no lawyer, but everyone needs to know some law and also chemistry to stay out of trouble.