Is it possible to convert Matcap colours to vertex or material colours? What I would like to do is simply to apply a Matcap while editing, and then use its base colour as the colour of the object for test rendering etc. Is this possible?

To put it another way, if the Matcap is lime green, how can I make the object lime green (using vertex or material colours).
This is not possible, using blenders built-in matcaps.

However you could setup a material that uses a matcap image that uses normal mapping relative to a camera object, then bake this to vertex colors.