There are a chain of contractual relationships between application for the mortgage and the closing. In terms of contract law it would depend upon whether you were a party to the contract or a third party beneficiary to the contract  (an intended beneficiary...not an incidental beneficiary) with the underwriter. In Connecticut it would require that you are specifically named in the contract as the intended beneficiary.

It is most likely that you are a party to the contract with either a VM or TC. You are probably not a party to the contract with the underwriter. Although as a business practice the underwriter may be interested in knowing that parties lower in the food chain are not being paid, and may consider that when electing to assign future business. You also have to be careful of a defamation suit coming back at you. In which case you may be entirely right, but would have to go to the trouble and expense of defending.

However, if you can show that the underwriter accepted your work when it knew or should have known that you expected to be paid for it...you may have a claim for unjust enrichment, but if he paid those lower in the food chain in an amount that included your fee it may give rise to a defense.

In terms of tort law you could try a claim for conversion (civil theft of your property..the abstract), but it is a long shot. You would have to show some tortious intent.

Some states have statutory remedies such as unfair trade practices or theft of services, but it does not seem likely that this would apply. You should also consult the law of the forum state to determine if it has made some form of relief available with respect to the underwriter that is not common to all states.

The problem would seem to be the remote relationship between you and the underwriter.

My money is on your side of the argument. If your client is circling the drain...sue him as fast as possible.