Hi Bert,
After assigning Purchase Agent to a user, no filter is made regarding Vendor Data.
The best option using standard D365BC, is to assign the user to a new Permission Set. Under this new Permission Set, we have several lines to detail all the permissions. For the lines that you want to apply a filter (I would say Vendor, Purchase Header tables), you should add a filter under Security Filter field.
This field allow to setup an filter regarding the content of the data (For example: Purchaser Code = AA).
Please keep in mind, that this security filter should be applied to all tables where you want to apply the filter.
Thank you!
Regards,
Jose Azevedo