To add an account number to a vendor account you must go to the vendor management page, located under the "Vendors" section of the Inventory tab.

Initially, your screen won't show any results.  You must either select a vendor from the drop-down menu or create a new vendor.

Once you've selected your vendor, click inside the "Vendor Account" field next to the desired location.  A text cursor will appear in the field to let you know you can begin typing.  Enter the account numbers and click on the "Submit Changes" button in the top-right corner of the page to complete your editing.