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Blinn-Phong shading model is based on lighting calculation per pixel, so it has to be implemented in the fragment shader.
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The specular light is based on the half-vector.
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According to the model, the intensity of specular light component is based on the cosine of the angle between the half vector and the normal.
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The intensity of the specular light Is is
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Is = ( N·H )shininess
where H is the half-vector
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In practice, GLSL formula becomes
float Is = pow( max( dot( N, H ), 0.0), shininess );
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Blinn-Phong light model is based on the half-vector:
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L – the incoming light vector
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N – the normal of the plane/pixel
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E – the vector from the pixel to the camera
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H – the half-vector: H = L + -E