I appreciate you coming back to the Community, ginahicks.
Thanks a lot for providing us the screenshots. Knowing so will help us isolate the problem.
The Sales Rep from the Lists menu can be added to sales transactions. However, as you've mentioned the sales rep on the purchase order was added as a custom field in the vendor's profile. Please follow the steps I've laid out below to edit it:
Take a look at this article for the detailed steps: Use QuickBooks Desktop custom fields.
That should answer your concern today.
Please let me know how that works you. I'll be here should you have follow-up questions. Enjoy your day.