Great thinking David!
When I first started customizing I would generally take a look at other screens and try and replicate their funtionality.
In many cases this can be tricky since Method has a number of moving parts and little triggers in places can often make a big difference across the entire screen.
The action you are looking to implement is called Retrieve Value from Table.
If you take a look at the Invoice screen you will be able to see that most of the actions which prefill the textboxes on the screen come from the Customer dropdown list.
If we explore these actions, there are two actions you should make note of:
1. Retrieve value from screen (Step 8 - Get resultCustomer Name)
2. Retrieve value from Table (Step 26 - Copy the terms)
In the Purchase Order screen you will have to replicate these two actions and customize them so they reflect the Vendor's name.
Workflow:
1. I made the Retrieve Value From Screen the first action and retrived the name of the vendor (In the vendor name dropdown located on the Purchase Order screen).
2. I then followed it up by retreiving the terms from the vendor table where the fullname was equal to the vendor name.
3. Lastly, placed the value into the Terms object on the screen.
Have fun customizing !