Maybe it helps to also see how these many terms are related and were they are coming from. (aka. Harald's little Christmas story of 2021 :nerd: )
Before skeletal animation, vertex animation (more correctly: per-vertex animation) was the way many (low poly realtime) characters were animated, where each keyframe stored the absolute location of each vertex of your character. These vertex positions were then interpolated linearly (along straight lines) during animation playback. Then animation transitioned to skeletal animation, some reasons are that vertex counts increased, and that bone rotation interpolation simply looks better than interpolating vertex positions on straight lines.
Then, as demand for deforming muscles (bulging when joints are bent) and face shapes on top of bone-based skeletal animation was growing, vertex animation was added back-in on top of existing bone-based skeletal animation. Luckily these deformations require only a single keyframe to be well defined, such as a bulging biceps shape when bent. This single frame vertex animation on top of skeletal animation was then called "blend shapes" or "morph targets" as a naming convention (and likely also as a marketing term to sell 3D editor features 😉 ). But in theory, "blend shapes" or "morph targets" are just single-keyframe additive vertex animation. In Spine terminology, vertex animation is called "deform keys", so if you create an animation with a single deform key, you have basically created a blend shape.
Hope this helped and didn't cause more confusion 🙂.
