How to register your manager as a vendor
From Protemos Wiki
To register your manager as your vendor, you can try a workaround using "plus addressing" email scheme where emails with "+" sign after email account username are delivered to the same email address.
For example, you invited your manager with the following email:john@example.com.
So, you should invite them as your vendor using john+vendor@example.com.
All the notifications will still be delivered to the main inbox - john@example.com.
Most email providers support such a workaround, but some of them do not.
You can always check whether it works with your email provider for the up-to-date information.
But in a nutshell, if your manager has received the invitation email when you invited them as your vendor, the workaround works.