I've worked in Illustrator for years... not professionally. I'm starting to charge for my services, and started wondering. I've taken an Illustrator class at my local college, and designed hundreds of things in Illustrator and other vector illustration software packages. But I've never seen, nor heard of, a best practices for creating layers within a vector illustration.
I recently asked an artist friend of mine for a high res image of one of his sketches so I can vectorize it. One of the things I would like to do once I am done is play with brush types on the image... however, if I do the digital inking like I usually do, then the image will be one huge vector image rather than pieces that I can edit and tweak.
Other than 'common sense' is there a guide for creating and using layers in a vector image program? Is there a book that would list out the types of pieces of an image that should be layered versus just one huge vector piece at the end?