1) Have an evoker summon a few vexes (spawned from eggs or with /summon won't die by themselves)

2) Use a nametag on a vex so its death message gets logged in console

3) Wait for it to die

4) Death message appears in console:

"Named entity died: <name> starved to death"

Vexes can't eat. A proper death message would be either something like "Vex vaporized" or just "Vex died." / "Vex died while fighting X."