The vertex paint cursor should work all over the mesh -- it is affecting the mesh vertex color, not the materials. The materials can read the vertex color but they should not have any bearing on what part of the mesh receives vertex color. Can you post a video or screenies?
I have a static mesh with multiple materials on it that I'm trying to vertex plaint but it seems the paint cursor only displays itself on one of the materials (and one that doesn't need the vertex painting effect, at that). Is this standard procedure? I'd have thought that vertex painting was little more than applying…
Ya sorry, here's what I mean. The beginning of the video shows that I can vertex paint, but the last half shows that the vertex paint cursor is only visible on the bottom strip (the first material). https://www.youtube.com/watch?v=_GtPoM3r0yA&feature=youtu.be