The editors suggest that the image is intended to represent the vault of a church decorated with the celestial spheres surrounding the empyrean.
In an age of great men he comes over as a second division figure, but the book is great fun and I am enjoying reading it.
It isn't the key to the Voynich manuscript, but it gives off the Voynich feeling of a deliberately created puzzle. I now think that this is the context of the VMS.