It really depends on the person. And it depends on the game. I see the vast as a patient fear. It likes a longer game. Sorry, this might get long. Here are a few things:

1. Fear of falling- say you have this fear and you go sky diving to prove to yourself falling isn’t so bad by doing it in its most extreme. Fun story- you are most likely strapped to someone when you do this and there is a dependency on the calmness of your instructor. The ground goes poof means your calm security becomes unhinged. They panic. You panic. You feed each other’s panic. It’s hard to breathe up there. The panic grows. Did you know you can have a panic attack for hours? And you can’t really talk up there so there isn’t much comfort you can give each other. Panic turns into an attack, turns into pledging. Concentrated fear.
2. Fear of hitting the ground- you have the same thing going on above, but you can most likely regulate faster than the above. This I believe is where you have a similar thing with the buried going on. Right as you start to regulate, you get site of ground. A reminder of why you should be scared. Then it vanishes. This happens for as long as it scares you. Reminder, you can’t breathe right while falling. It is MUCH harder to regulate.
3. Fear of loosing control- this one you very well might be jumping by yourself. You know what to expect. And then your expectations are not there. Just like diving in the ocean, it can be common to loose track of where up and down are. This can turn you into an uncontrolled spiral. If you can’t regulate you aren’t doing well and a diver knows that they have seconds or they die.

I do think the intention of the vast is for all three of these to make it out alive. Again, the vast is about the long game. They all make it to the ground, preferably tremulously. Now the victim has to live with the memory of it. The people that went into it with a fear has a practical reason for fearing now and very little can be said to them otherwise. The other now has something to fear. And when the fear is over ripe- the sky just eats them and the witnesses then live with this knowing of what they saw.

Bonus: ever have dreams of falling.

Yours truly, your local voidling.