Another airship - small issue with vertex paint

I started another airship project last week; i’m making fairly good progress with the model and textures:


One issue that’s annoying me though is vertex paint - as you can see, i’ve darkened the areas of the hull where the appendages meet it, so simulate accumulated oil and dirt. What’s bugging me is the superstructure - i’ve used the same settings, but I can’t get the painted vertices to show up when rendering.
I’m using 2.69 with blender render.

“Vertex colour paint” is selected in the materials tab, which is usually the issue in this case.
Anyone have any ideas?