GLSL - make effect only on one texture

Newbie & Debugging Questions
#1

I wish to blur my background, with the following fragment shader:

 varying vec4 vColor;
    varying vec2 vTexCoord;

    uniform vec2 screenSize;

    uniform sampler2D u_texture;
    uniform vec4 v_time;

    const float RADIUS = 0.75;

    const float SOFTNESS = 0.6;

    const float blurSize = 1.0/1000.0;

    void main() {

	    vec4 texColor = vec4(0.0); // texture2D(u_texture, vTexCoord)
	    texColor += texture2D(u_texture, vTexCoord - 4.0*blurSize) * 0.05;
   	    texColor += texture2D(u_texture, vTexCoord - 3.0*blurSize) * 0.09;
  	    texColor += texture2D(u_texture, vTexCoord - 2.0*blurSize) * 0.12;
   	    texColor += texture2D(u_texture, vTexCoord - blurSize) * 0.15;
   	    texColor += texture2D(u_texture, vTexCoord) * 0.16;
   	    texColor += texture2D(u_texture, vTexCoord + blurSize) * 0.15;
   	    texColor += texture2D(u_texture, vTexCoord + 2.0*blurSize) * 0.12;
   	    texColor += texture2D(u_texture, vTexCoord + 3.0*blurSize) * 0.09;
   	    texColor += texture2D(u_texture, vTexCoord + 4.0*blurSize) * 0.05;

        vec4 timedColor = (vColor + v_time);

	    vec2 position = (gl_FragCoord.xy / screenSize.xy) - vec2(0.5);
	    float len = length(position);

	    float vignette = smoothstep(RADIUS, RADIUS-SOFTNESS, len);

	    texColor.rgb = mix(texColor.rgb, texColor.rgb * vignette, 0.5);
		
	    gl_FragColor = vec4(texColor.rgb * timedColor.rgb, texColor.a);
    }

But the problem being is that the shader blurs all screen… and i want it to blur only the background texture… what should i write to make it focus on my background texture instead of all the screen?