When you are creating vendor notifications and you are follow the steps in TechNet article “Manage notifications to vendors” - http://technet.microsoft.com/EN-US/library/hh242691.aspx

You may receive the following error message: "There is no valid vendor contact person recipient for this message. The notification was not sent." if you have not linked a
contact from the vendor to a user.  Below are the steps to link the contact to a user.

 Find the Party ID on the Vendor.

  1. Go into the Vendor form – Accounts Payable | Common | Vendors | All Vendors
  2. Double click on the vendor in the All Vendor list page
  3. Click the Change Party Association button in the General Fast tab

       4.  Close the Vendor form

Create a contact on the Vendor.

  1. Go into the Global Address Book – Home | Common | Global Address Book
  2. Find the Vendor account using the Party ID in the Global Address Book list page and select it

     3.   Click the Contact button in the New group of the Global Address Book tab

    4.    Make sure the Contact for is the vendor you are looking for
    5.    Enter the Name of the contact


     6.     Close the form

 Link the Contact to the User

  1. System - Administrator | Common | Users | Users
  2. Select  User
  3. Click the Relations button in the Setup group of the Users tab

     4. Click the New button

     5. Find the contact you created above

     6. Tab though the fields

     7. Click the Add button in External Relations

     8. Select Relation Type: Vendor

     9. Select the Vendor Name you linked the contact to


     10.   Close the form

If you have questions on fields in the Vendor Notifications form you can view TechNet article: “Vendor notifications” - http://technet.microsoft.com/EN-US/library/hh242813.aspx

If you have questions setting up a contact and user in Enterprise Portal you can follow the steps in TechNet article “Manually set up user access to the Vendor portal [AX
2012]” - http://technet.microsoft.com/en-us/library/hh209340.aspx