When you summon Vexes, they constantly fly around and through blocks, even bedrock! But for map-makers, this might be just too inconvenient to work with. So I have figured out how to summon Vexes that cannot go through blocks and escape an area!

The idea here is to summon the Vex riding an invisible mob. As long as the mob the Vex is riding isn't another Vex, your Vex won't be able to escape an enclosed area. If the invisible mob is a flying mob such as a Phantom, the Vex could still go through blocks when the Phantom flies right up against a ceiling, but because the Phantom can't go through blocks, the Vex won't be able to fly away completely so is still quite contained in your area.

Basic Example Command:
/summon endermite ~ ~ ~ {Passengers:[{id:vex,PersistenceRequired:1}],Silent:1,PersistenceRequired:1,ActiveEffects:[{Id:14,Amplifier:0,Duration:999999,ShowParticles:false}]}
- PersistenceRequired prevents the mob from despawning
- I chose to use an Endermite because it's hitbox is about as wide as a Vex (Endermites do 'drown' if they touch soulsand though, so this might not work for you)
- I also gave the Endermite a 'Silent' tag so it won't make noise and give away the fact that the Vex is riding an Endermite



Customising:
- Putting the Vex on a passive mob will weirdly result in the Vex not being able to move at all. So if you want the Vex to chase after you, summon it on a hostile mob.

- If you don't want the Vex to fly up and down, simply summon it on a mob that can't fly.

- By default, the summoned Vex won't be holding the iron sword that natural Vexes spawn with. The below command shows how to give mobs items (here the Vex is holding an iron sword and a shield):
/summon vex ~ ~1 ~ {HandItems:[{Count:1,id:iron_sword},{Count:1,id:shield}]}

- If the mob the Vex is riding is invisible and silent, it doesn't really matter what this mob is (unless you care about it's health, or weaknesses like how Endermen can't touch water), so you could choose the mob based on how high it's hitbox is and so choose how high the Vex glides off the ground.

- To make the Vex ride a baby mob, here are some example commands:
/summon zombie ~ ~ ~ {Passengers:[{id:vex}],IsBaby:true}
Use the IsBaby tag for mobs that have a baby variant that doesn't grow up - baby zombies don't grow up into adults, so if you want a baby zombie, use the IsBaby tag and set it to true.
/summon villager ~ ~ ~ {Passengers:[{id:vex}],Age:-2147483648}
Use the Age tag for mobs where the baby does grow up eventually - mobs like sheep, turtles, and villagers. There is no way to just turn off a mob's age increasing, so using the maximum number possible is the best way. When the number reaches 0, the mob grows up, and the number is in ticks (there are 20 ticks per second), so for -2147483648 to reach 0 it would take 1300 years.... That should be good enough for your map :p




Command I Used:
/summon endermite ~ ~2 ~ {Passengers:[{id:vex,Invulnerable:1,PersistenceRequired:1,Silent:1}],Invulnerable:1,PersistenceRequired:1,Silent:1,ActiveEffects:[{Id:14,Amplifier:0,Duration:999999,ShowParticles:false}],Attributes:[{Base:0.175,Name:"generic.movementSpeed"}]}
This command makes the Vex and Endermite invulnerable, silent, and undespawnable. The Endermite is invisible, and has a movement speed of 0.175, which made it move slowly.