sent letter, now what

You dispute with the credit reports. If they come back verified then the collection agency has violated your rights because they are not allowed to verify anything while under a DV. You could then take them to court for just those violations. Also, when the CRA's verify you can send them a certified letter asking for them to explain their procedures on how they verified because the debt in question is unvalidated (send them a copy of the letter you sent the CA)

You can use these violation to send another letter to the CA and point out that they violated your rights by verifying an unvalidated account and that they are guilty of reporting false information. Give them like 15 days to remove the account or you will file suit. Depending on how the CRA responded if you do sue, you can name them as well since they allowed this unvalidated information to remain after you disputed.