Doc talk:2.6/Manual/Materials/Multiple Materials

This page talks about assigning materials to vertices, but it appears that the material assignment is to faces, not vertices. Note that the material index property is on the MeshFace object here: http://www.blender.org/documentation/blender_python_api_2_60a_release/bpy.types.MeshFace.html#bpy.types.MeshFace

The MeshVertex object has no materials index (it can have a colour though): http://www.blender.org/documentation/blender_python_api_2_60a_release/bpy.types.MeshVertex.html#bpy.types.MeshVertex

Also, it is not possible to have several halo materials rendered on one mesh. The render picks up only the first material slot.