Your employer cannot legally take any adverse action against you because you failed a CVSA "test." For it to do so would be a violation of the Employee Polygraph Protection Act (EPPA), which applies to all forms of "lie detection," and not just polygraphs:

http://www.fas.org/sgp/othergov/polygraph/eppa.html

Although I'm not a lawyer and not in a position to dispense legal advice, I think it would be wise at this point to document everything that has happened thus far and continues to happen in the coming days and weeks. Were you legitimately "written up" twice since the incident? It's possible that your employer is seeking a pretext for terminating you, since it can't legally do so based on the results of a lie detector "test" alone. If you later decide to bring a lawsuit under the EPPA, contemporaneous notes and other documentation will be helpful.

Even if you don't have the money to retain a lawyer at this time, it would probably be a good idea to at least seek a consultation with one who has a background in labor law. If you don't know any such lawyers, the Martindale Lawyer Locator service may be of assistance in finding one near you:

http://www.martindale.com