Steve:

Both would have to be suggestions.

Having the ability to have separate auto number by document type would be a useful feature.

I have no idea how you would restrict users to specific vendors, maybe by using the IC Item Primary and Secondary vendors so that if you order item X you can only order from one of the vendors assigned to item X. Then use security (or even a template) to restrict access to the vendor field so that the user can only select one of the assign vendors.