Part of the problem is that the lens effects do not change the alpha, they only work on the RGB channel - something that we probably need to address at some point.
The other part of the problem is that the VFB shows the image with sRGB color correction whereas the Bitmap viewer in 3ds Max uses gamma 2.2; the two are very very similar except for very dark parts of the image, like in your case. If you duplicate the VFB image to a 3ds Max bitmap, do you get the same issue?
Best regards,
Vlado