The bug
The Vex kills a witch when the witch harmed the evoker or the vex.
Attachments
Comments 3
That's not really a reason, jhonnies attack all but illagers, vexes are no illagers.
Vexes should also not attack non-illagers (which they do), but the witch is currently not marked as an illager.
Probably WAI as per the resolution of MC-108403.