The VS EU Active metric represents the percentage of overall GPU time that the execution units (EUs) were actively executing Vertex Shader instructions. This metric is important if vertex processing seems to be a bottleneck for selected rendering calls.
If VS EU Active is 50%, half of the overall GPU time was spent actively executing Vertex Shader instructions.
If VS EU Active is 0%, no Vertex Shader was associated with the selected draw calls, or the amount of time actively executing Vertex Shader instructions was negligible.
To improve performance:
If VS EU Active accounts for most of the EU active time, then to improve performance you should simplify the vertex shader or simplify and optimize the geometry of your primitives.
If VS EU Active is significant, you should examine your vertex shader code to find the reasons that might be causing stalls.