Can I use Vertex paint with volume shaders? Simple question: Can vertex paint be used with volume shading in Cycles?

When you plug the vertex-colors into any volumetric node (emission, scatter absorption) and plug the output of that node into the volume input of the material output node the material will not render.

Is this behaviour a bug or am I doing it wrong?
Unfortunately, AFAIK this is not possible currently.

This is because the vertex colors are 2D and can't really be represented in 3D. You'll notice that the same thing happens with image textures.

Instead you could try adding the vertex colors on top the volumetrics with a transparent shader, but I'm not sure this is really what you want.