The system will only show a Vendor Name when the AI can identify something it perceives as a vendor name. Secondly, it needs to be able to identify something it perceives as an invoice number. When it cannot, it should the sender email address instead of trying to guess the vendor name.
Sometimes this is because the email in not an invoice at all. See Delete Bogus Sender Lines for how to examine and handle these lines.
If the email is an invoice, you will be able to easily choose the correct vendor and you may need to provide your own version of an invoice number to post this in Dynamics GP.