The VS Invocations metric shows the number of times a Vertex Shader was invoked.
The number of vertex shader invocations depends on both the vertex and primitive counts and the operation of the post-transform vertex cache (VCache). In an optimal situation, the GPU fetches already-processed vertices from the cache rather than recalculating this data, which could impact the value of this metric.
To improve vertex processing performance and reduce the number of vertex shader invocations, try to reorder the geometry for optimum VCache usage.
If you render point sprites, this metric is always equal to Vertex Count and Primitive Count (that is, no optimizations are necessary).
The size of the VCache varies for different GPU models, so you might see different metric values when using the same geometry on different hardware.