possible blending factors per vertex


I’ve writen a terrain renderer with DirectX 9. The terrain is devided into several levels. Each level has got a texture. A triangle of the terrain mesh is influenced by no more than 3 textures. I use diffuse (primary) and specular (secondary) vertex color alphas to do the blending between 3 textures with multitexturing.

I want to port this to OpenGL. In the docs of ARB_texture_env_combine I read secondary color alpha is not available as input. How can I port the DirectX code to OpenGL then? Do I have to use some vendor specific extension?