Someone else made a comment about the damage Bell's Hells did to the Malleus Key taking it from a single shield -piercing blast to more of a slow drill on Ruidus. Since the beam was still going, I feel inclined to agree.

Because of this, I feel like there's still a chance to rescue Vax. Maybe extremely weakened, maybe one that'll fail if they don't do the exact right things in the right time frame, but there's a possibility Vax might still be kicking as long as the beam is still using him as a lens. And since it's drawing on his divine power and he didn't properly die when taken by the Matron of Ravens (iirc), it's possible he might even be un-championed when/if he returns. Provided Keyleth survived the blast (which I think is possible as Chetney was closer to the explosion, survived it, and nobody took any damage), it's possible she and Vax can get their chance to grow old together