Hello Ed,
The dropdown you are refering to is based on UsersManager field on the ExpReportUserDetails table. The table itself is a relation table because it holds no unique fields; All the fields are from other tables. The UsersManager field is from the Users table. This is why you are not seeing Vvendors in the list because they are not in the Users table. Portal users are based on existing tables such as users, customers, vendors, etc. I'm assuming your portal is based on either the contact or vendor table.
To make the change, you will have to change the UserManager field to be based on another table. The problem is that you will affect any screen using the ExpReportUserDetails table and it could create problems. Also, if you created a new field, you would have to find references to the UsersManager field and change it to your new field.
I hope this helps,
Greg