Welp I sent off another DV to them, basically the same thing, but mentioning that they failed to respond to the first DV within 30 days, and that if they fail to respond to this one within 15 days, I will assume that they are unable to validate the debt and will take steps to protect my rights (i.e. start pestering the CRAs). Also included copies of everything up to this point.
We'll see what happens now