In the Customer Centre there is currently no Vendor ID field. When dealing with businesses, even if you are the supplier they often require a Vendor ID from you on all paperwork. A field that feeds through to invoices, estimates, receipts would be handy so that information would be part of your customer's profile.
It would be handy to have a field similar to the 'Account Number' function in the Supplier Centre but that it feeds through to your forms for that customer.