"When will your time be served? When will your debt be paid?"
Keyleth keeps thinking about Vax being indebted to the Matron. And that makes sense, because it is a way to explain why he left her. If he had no choice, there is no reason to blame him, and instead she can blame the Matron for keeping him away.
Except I don't think Vax is still indebted to the Matron, at least not in the way that Keyleth is implying. Vax wilfully followed his God. He wilfully upheld the things the Matron stands for - one of them accepting death as part of life, even his own.
How can Vax allow himself to be saved? How can he do that and not revoke anything he has learned to accept through worshipping the Matron?
Keyleth has to accept that it was Vax's choice. That is wasn't the Matron who killed him, but Vecna, and that Vax was not willing to be resurrected.
So she can finally be angry with Vax. So she can finally be angry with the situation. And so she can finally start to heal.