The default United States country value must be used for vendor addresses in order for Aatrix to recognize the address as a U.S. address so that it pulls in the State and Zip Code.  Alternative or duplicate country values for the United States will not be recognized by Aatrix as U.S. addresses.

To correct the vendor's address:
  1. In Accounts Payable go to Records > Vendors
  2. Open a Vendor record with the issue
  3. Go to Addresses and open the default 1099 address
  4. Update Country to the correct United States value (if some vendors are pulling in the State and Zip Code through Aatrix, these vendor records can be checked to determine which value to use)
Note: Country values are setup in Configuration > International.