On Rage128, Radeon 7000, Radeon 7200/7500, Geforce 3, Geforce4MX, Geforce4Ti vertex program are software emulated (MAX_PROGRAM_NATIVE_INSTRUCTIONS_ARB = 0)
Since GLSL is shipping for Apple Software renderer, why not enabling GLSL with vertex shader only for thoses renderers ?
Like having GL_ARB_shading_language_100 and GL_ARB_vertex_shader but not GL_ARB_fragment_shader.
Performance wise, it would perform at the same speed than GL_ARB_vertex_program, but using the high level language benefit.
Of course, for GeforceFX, Radeon 8500 or better, thoses extensions won’t be enabled.