Thank you for your post.
If your drop down is based on a numeric field with values to display from a second field, then if you plan to enter the information, you must enter the primary field; not the secondary field. This is why you are getting the error message.
If the name of the Vendor is truly unique, then you may want to use this as your "key" field. The downfall is the Vendor name must be EXACT. For example, "FileMaker, Inc." and "FileMaker Inc." are two different entries. Also, if the name of the Vendor changes due to merging with another company or name change due to marriage, this would affect prior records.
You can create a second table occurrence where the primary field still exists, but the secondary field is used for the Vendor name.
If you need clarification for any of the above steps, please let me know.
Thanks for your quick reply. I was a little afraid of that. With many entities with very similar names (or multiple nicknames), relying on the name to be the primary key is problematic. I suppose a read-only layout for browsing and finding, and buttons to open layouts to edit or enter new data in a user-legible format may be the way to go.