The bug

Vex do not chase their targets they're hostile towards, unless they're actively in their "attacking" mode where their charge their target and strike them. They follow and track their opponent in this mode, chasing them down until they can get a hit or they avoid it. However, when in their "waiting" state where they're still actively focused on a target and could charge at any random time, although they will look at whatever they're attacking, they won't follow it.

This means you can easily just sprint, or even walk away from a Vex and they won't follow you and easily give up, as demonstrated in this video:

VexRangePlayer.mp4

This is especially noticeable when an Evoker tries to attack a villager; the Villager simply runs and they Vex do not follow unless they're in their "red" state, as can be seen here:

VexRangeVillager.mp4

This means that Zombies, Pillagers, Ravagers, and Vindicators are infinitely better at chasing villagers.

How to reproduce

1. Spawn an Evoker in an open field and have it summon some Vex, or just summon few Vex yourself
2. Run, or even walk away from them
3. They will follow you at first as they charge at you, but even though they will still be tracking you, they won't chase after that and very easily give up
4. You can also try this with a Villager; the villager can very easily just walk away