So, as we know, in S4, The Whispered One (aka Vecna) had just ascended to Godhood with a master plan I put in my top 10 master plans of all time (still can't beat Palpatines sadly) but anyway he expertly manipulated everything so no matter what choice Vox Machina made, they would fall right into his hands, and he just ascended to Godhood.

But Vax was considered his greatest threat and one notable line from Vecna was "Thanks for removing his new gods powers" (not exactly but basically)

Vax spit in Vecna's face as a act of defiance and so Vecna (who has a huge ego) killed him.

This is a perfect way to set up a villains flaw, like how Gus Frings major flaw was his desire for revenge or Sauron's was his inability to fathom selflessness, Vecna's ego is his flaw, and to set a point, he killed Vax, to where Vax is a servant of the Goddess of Death aka the Matron of Ravens, unknowingly sending him back to her and likely meaning he'll get those pesky powers Keyleth took from him that could be a threat to Vecna back most likely.