Hello, Zack. Welcome to FileMaker.
Your need is fairly common. Often referred to as a "list-detail" view, it involves capturing the key field of the portal row in a global field, which is then used as the key in another relationship. Basically, you follow these steps:
1) In your parent table (the one where you show the portal from), create a global field of the same type as the key field in the child table (the one where the rows appear).
2) On your Relationships Graph, create a relationship from that new global field to the table where your other information occurs (in this case, vendors).
3) Create a script that grabs the key from the portal row and sets the global field equal to that value, then refreshes the window.
There are some additional details (like what do you do when the user first navigates to that screen? What do you do when the user switches records?), but that is the essence of the technique.
Just to expand on what Mike says above, you basically have two separate relationships, one to show the portal row and one to show the detail of your selected row.
one to many = displays portal rows
one to one = displays your detail for the selection
You will likely have a button on the portal row that sets the necessary key field to relate to the record for the detail view (your one to one). This might be a global, but doesn't have to be, depending on your needs. Also, the button doesn't need to be attached to a script, it can set the key field in the button settings, but you might want to organize it into a script in case you want to do anything else (call other script steps to do anything else).
Hope this helps.
Thank you very much for your ideas.