If you already know this ignore it but if you don't, then know that if's in materials stages turn on and off those stages, so you need to make sure you are setting parm11 to a valid value above zero in a script somewhere.
But if removing those, didn't solved the problem, then the problem could be the vertex colors itself, are you sure the model has the correct vertex color info on it? Afaik the engine only supports grayscale vertex colors in the RGB format no alpha.
Also instead of blend diffusemap keyword try the blend add (gl_src_one gl_dst_one) not sure if this matters but the basic example for vertex colors in this link uses it.
I haven't used vertex blending for a very long time so all of this is rusty on my mind unfortunately.
Also not sure what you mean with ""lawn_vertex_blend" is the vertex-blended DDS image file exported from Blender and is also referenced in the .ase file." but a material name like yours "textures/darkmod/map_specific/lawn_vertex_blend" shouldn't be a link to a real texture, in reality that is just a virtual path to a fake folder that DarkRadient uses to display in the media section of the editor.
You can have a material with just a single word on its name and it will show in the media tab, just not inside a folder but in the global space.
it is like this:
virtual path/material name { material code } or material name { material code }