The following article outlines how to add Vendors manually. Only users with Manage Vendors permission under Procure will be able to perform the steps listed.
Go to Procure → Vendors.
Select the New Vendor on the right side of the page.
Fill out the details.
Don't forget to Add Vendor when you're done.
If you need to add multiple vendors at the same time, please follow the steps on How to Import Multiple Vendors.