Keyleth fundamentally misunderstands Vax's service to the Raven Queen, which is not to say I blame her at all! The Matron has not bound Vax to pay a debt, there is no "payment". Vax chose to serve the Matron. The circumstances of his death were unexpected and terrible, yes, but the Matron did not steal him. Keyleth (and Vex) still thinks he is paying her back for saving Vex. Vax spent years coming to terms with what serving the Matron would mean and *he chose it* in the blood pool in Vasselheim.
But how can Keyleth come to her own terms with his choice if he keeps sending her ravens? How is she supposed to know what that means?
@mareastrorum hit the nail on the head