for some reason a few vertices don't deform correctly. The vertex value is at 1, I only have 1 vertex group for the mesh, there shouldn't be anything else affecting the vertices. normalizing weights doesn't help the issue also occurs when weights to multiple bones are applied to the vertices, i just tried applying a single weight for troubleshooting.
Yup, bendy bones.
We can see what bones have bendy bones by changing the display to Bendy Bone, and disabling shapes. (Man, I hate shapes, extra work just to give the animator less information.) When we do, we see that the bone to which your chest/vest is weighted is controlled by bendy bones:
So are quite a few other bendy bones. Which you'll want to be aware of for further work.
To change a bone to stop being bendy, you just have to change the segment count to 1, in properties/bone/bendy bones/segments.
I'm not into Rigify, but I've seen a lot of problems from Rigify users related to bendy bones. It appears that there was a recent change that made almost everything bendy, and that it's causing a lot of users problems. There should be some way to prevent this from happening if you do some searching on Rigify documentation (bendy bones are not an unalloyed good, as demonstrated here.)