No, just change the mathematical formula to go brighter outwards. Try modifying the vignette shader like this:
float vignette = 1.0 + 0.4 * (dot(v_texcoord0 - 0.5, v_texcoord0 - 0.5) * 2.0);
In case the vignette starts further out in FF, maybe something like this might work:
float distance = dot(v_texcoord0 - 0.5, v_texcoord0 - 0.5);
distance = clamp(distance - 0.7, 0.0, 1.0);
float vignette = 1.0 + 0.7 * (distance * 3.0);
Or something, just guessing the numbers. Play around.
float vignette = 1.0 + 0.4 * (dot(v_texcoord0 - 0.5, v_texcoord0 - 0.5) * 2.0);
In case the vignette starts further out in FF, maybe something like this might work:
float distance = dot(v_texcoord0 - 0.5, v_texcoord0 - 0.5);
distance = clamp(distance - 0.7, 0.0, 1.0);
float vignette = 1.0 + 0.7 * (distance * 3.0);
Or something, just guessing the numbers. Play around.

