How do I file a lein on someone we did a job for them and they are not happy with the fact that we will defraud the insurance.

Did you do more than look at the vent cap? If you did work which you were contracted to perform, you should be paid. You may not like what I have to say next, but I would rather be honest with you than tell you just what you want to hear. I do not see any basis for an insurance fraud claim based upon the information provided in your question. There may be more to it that would change my opinion. However, if the property owner sustained some damage, and it was soley cosmetic, and not effecting the funtion of the vent, they may still be covered for the cosmetic damage, and be entitled to have it replaced. Like cosmetic damage to a car with collision coverage, the policy may entitle them to have the home restored to its prior state cosmetic or otherwise. Technically, it is not your job to determine what they are entitled to under their contract of insurance. Insurance companies are always looking for ways to save money, and they may have tried to use you to do so in this instance. It also sounds like the home owner is not happy with you for getting in the middle and effecting their claim. Unfotunately, now they are refusing you the payment which you deserve, and now you must fight for it. To do so, you have some choices, including the filing of a a mechanics lien. Liens, can apply pressure, but are not always worth while. They do not entitle you to payment where there is a dispute. In other words, you still have to prove your claim is valid. Mechanics liens, will provide some security to contractors in the event of a loan closing or sale of the property before a case can be heard. In the event you do win, you then have the property as collateral. Mechanic's liens also have to be done carefully, exactly as is required by state statute, and they do not last forever. You should seek the services of a local attorney about whether it makes sense for you to file a lien, or take other action, and have him/her do so for you.