The metric VS EU Active % represents the percentage of overall GPU time that the EUs were actively executing Vertex Shader instructions.
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 that the amount of time actively executing Vertex Shader instructions was negligible.
This metric is important if vertex processing seems to be a bottleneck for selected rendering calls. 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 reasons that might be causing stalls.
Inspect the shader code in the Graphics Frame Analyzer.