Vignetting is a reduction of an image's brightness or saturation at the periphery compared to the image centre.

Pass 1:  Render the scene to texture using an FBO
Pass 2:  Render a full-screen quad using the texture from pass 1 to the default framebuffer.  In the fragment shader,

• Compute the distance from the centre of the screen.
• Darken the fragment based on this distance.

Code below inspired from Github

// some things above omitted
void main()
{
   vec4 texColour = texture(sampler0, vTexCoord);
   vec2 vectorFromCentre = vTexCoord.xy - vec2(0.5);
   float vignette = 1 - smoothstep(0.3, 0.75,       length(vectorFromCentre));  
   texColour.rgb *= vignette;
   vOutputColour = texColour;
}

High