Hi Casey,

Assuming you have an external table connected that has the Vendor name and number information, you only need to create a template and define the dynamic field information,  the key to auto filling the Vendor number is using the parent field to key off the vendor name.

I have a database table connected called 'vendor' with two columns,  vendorName and vendorNumber.

First I create my template with my metadata fields in the General Tab:

 

Next I configure the dynamic field tab,  first with a lookup on the vendorName and 2nd the vendorNumber with the Vendor Name as the parent field to key off of.

Now as I fill out the template, once the Vendor Name is unique, the vendor number will autofill.

Hope this helps get you started.