Keep all responses from all CA's. By law you have 30 days to respond to the first letter from CA which it appears that you have done. If you receive any additional letters, don't ignore them. ALWAYS respond, and ALWAYS make sure you receive full validation of account from the CA.
I prefer using this letter from Happy Days for DV's. If they can't validate , they can NOT report to CRA's .
If they report after 35 days from receipt of your DV and no response to you, they are in violation of FDCPA, and can be sued.
The only problem with DV before SOL is if they CAN FULLY VALIDATE, you face the possibility of a lawsuit, and a judgment being entered against you. You really should do whatever you can to avoid judgment, because this can be a major Derog on CR for up to 10 years after judgment is entered.