The Bug:

While idley flying about, unlike most other mobs (most similarly compared to the Allay), they are almost always constantly on the move. They hardly ever stop moving, constantly changing directions. 

This erratic movement itself may be WAI.. however,

They DO occasionally sometimes stop, but unlike every other mob, this "stop" is unusually short, lasting at max about 1 full second.

This behavior can be observed in the videos below:

VexIdle.mp4

VexQuickStop.mp4

This seems like unintended behavior, as Vex are the only mob to have an inconsistent amount of time they stop moving compared to most if not all other mobs. It's almost like they're trying to stop moving but something in their AI prompts them to change directions, which leads to them moving again, creating strange very short pauses.

How to Reproduce:

1. Summon many Vex and take note of their flight pattern.
2. Notice how they almost never stop moving, which is unusual for most mobs
3. That may be WAI, however they do sometimes TRY to stop moving, but are forced to move again, creating weird very short pauses